What is Dockerfile

docker image is created from docker file which runs inside container.

Dockerfile Instructions

Name Meaning
FROM This is the first instruction in every Dockerfile. It specifies the base Docker image
FROM python:3-slim This is an official Python image from Docker Hub.
It includes Python 3 and is a smaller, slimmed-down version compared to the full Python image.
Slim images have fewer packages installed by default to keep the size small, but they still provide the essential tools to run Python applications.
WORKDIR sets the working directory inside the container
WORKDIR /app Means create /app inside container.
Any subsequent COPY, RUN, or other commands in the Dockerfile will operate relative to this directory
COPY src dst Copy files or directories from your local file system (the host) to the Docker container's file system.
COPY requirements.txt ./ Copy requirements.txt from your local machine to the current working directory (/app, as set by WORKDIR)
./ means the file is placed directly inside /app (the working directory).
RUN Execute a command in the Docker image at build time.
CMD Command which is run at startup with DEFAULT arguments

$ Dockerfile
CMD ["echo", "Hello World"]

$ docker run my_image
Hello World

CMD can be easily overridden by adding arguments at the end of docker run

$ Dockerfile
CMD ["echo", "Hello World"]

$ docker run my_image ls    //Hello World is not printed
-list of files-
ENTRYPOINT This command specifies the default command that will run when the container starts. Mainly used to set the main executable for the container. Can be overridden using --entrypoint flag

// Run entrypoint.sh script on startup
ENTRYPOINT ["/entrypoint.sh"]


Most Often Use case: Using ENTRYPOINT, CMD together

$ Dockerfile
FROM python:3.9
ENTRYPOINT ["python", "app.py"]
CMD ["--port", "8080"]

// Standard run
$ docker run my_image runs
python app.py --port 8080

// Custom run
$ docker run my-app --port 9000 runs 
python app.py --port 9000
EXPOSE (only Documentation) The EXPOSE command is used as documentation to inform users and other tools that the container will listen on specific network ports at runtime.
It DOES NOT actually publish or open the port to the host machine or the outside world.

PUBLISH (-p): There is not publish command in Dockerfile, but if you want to open a port then you can specify using -p after docker run

// command maps port 80 on the host to the exposed port 8080 in the container. 
docker run -p 80:8080 my_image_name

Dockerfile Example

We will use the code from this repository https://github.com/meobilivang/newsreader-cli
Step Description
Step-1: Clone the code 

cmd> git clone https://github.com/meobilivang/newsreader-cli
cmd> cd newsreader-cli
Step-2: Create run_test.py file in root of cloned repo 

#!/bin/bash

set -euo pipefail
cd /app
pytest
Step-3. Dockerfile content. Create Dockerfile in root of cloned repo 

# Download python-slim image from docker hub
FROM python:3-slim

# Create /app directory if not present.
# All COPY, RUN commands will be executed relative to WORKDIR.
# Set the working directory
WORKDIR /app

# We are inside /app. Copy requirements.txt from your local machine to the current working directory (/app)
COPY requirements.txt ./

# RUN executes a command in the Docker image at build time(inside app directory)
RUN pip install --no-cache-dir -r requirements.txt

# copies the entire contents of your current directory (cloned repository on your host machine) into the /app/
# COPY src(everything from present dir) dst(/app folder in docker image)
COPY . /app

# RUN executes a command in the Docker image at build time
# Install the pytest inside container
RUN pip3 install --no-cache-dir pytest
RUN pip3 install --no-cache-dir py==1.11.0

# Install the package
# either a setup.py or pyproject.toml file need to be present in root directory of the project.
# RUN pip install --no-cache-dir -e .

# Run test cases to ensure everything is working
# run pytest inside docker container
# pytest should look for tests in the /app/tests/ directory.
# --maxfail=1: Tells pytest to stop after the first test failure
# --disable-warnings: Suppresses any warnings, so they are not shown in the test output.
RUN python -m pytest /app/tests/ --maxfail=1 --disable-warnings

# This changes the working directory back to / (the root directory) after the tests have been run
# This ensures that the next commands or any processes executed will be run from the root directory of the container, not /app
WORKDIR /

# copies the run_tests.sh script from your local machine into the root directory (/)
COPY run_tests.sh ./
RUN chmod +x /run_tests.sh

# This is the command specifies the default command that will run when the container starts.
# /bin/bash: Specifies that a Bash shell should be used.
# -s: This tells Bash to read from standard input.
ENTRYPOINT ["/bin/bash", "-s"]

Create Docker image

Create docker image using docker file created in above step

> docker build --platform linux/amd64 -t newsreader-cli-test .
[+] Building 33.0s (15/15) FINISHED                                                                             docker:desktop-linux
 => [internal] load build definition from Dockerfile                                                                            0.2s
 => => transferring dockerfile: 886B                                                                                            0.1s
 => [internal] load metadata for docker.io/library/python:3-slim                                                                1.8s
 => [internal] load .dockerignore                                                                                               0.1s
 => => transferring context: 2B                                                                                                 0.0s
 => [ 1/11] FROM docker.io/library/python:3-slim@sha256:c24c34b502635f1f7c4e99dc09a2cbd85d480b7dcfd077198c6b5af138906390        0.0s
 => [internal] load build context                                                                                               0.1s
 => => transferring context: 5.16kB                                                                                             0.1s
 => CACHED [ 2/11] WORKDIR /app                                                                                                 0.0s
 => CACHED [ 3/11] COPY requirements.txt ./                                                                                     0.0s
 => CACHED [ 4/11] RUN pip install --no-cache-dir -r requirements.txt                                                           0.0s
 => [ 5/11] COPY . /app/                                                                                                       12.9s
 => [ 6/11] RUN pip3 install --no-cache-dir pytest                                                                              4.1s
 => [ 7/11] RUN pip3 install --no-cache-dir py==1.11.0                                                                          3.1s
 => [ 8/11] RUN python -m pytest /app/tests/ --maxfail=1 --disable-warnings                                                     2.4s
 => [ 9/11] COPY run_tests.sh ./                                                                                                0.1s
 => [10/11] RUN chmod +x /run_tests.sh                                                                                          0.6s
 => exporting to image                                                                                                          6.9s
 => => exporting layers                                                                                                         6.6s
 => => writing image sha256:5f77f59a7ebf7e5c4357e0c1532cac0cd417e1f2a2700c7d8bf757a89a21f9cd

Check Docker Image

Now we have created docker image, its time to go inside it and check the contents. 

> docker images
REPOSITORY            TAG       IMAGE ID       CREATED         SIZE
newsreader-cli-test   latest    5f77f59a7ebf   7 minutes ago   734MB

// Check Architecture
> docker inspect --format='{{.Os}}/{{.Architecture}}' 5f77f59a7ebf
'linux/amd64'

// Run tests inside docker container
// Check run command on Commands page
> docker run --platform linux/amd64 -i newsreader-cli-test < pytest
============================= test session starts ==============================
platform linux -- Python 3.12.5, pytest-6.2.5, py-1.11.0, pluggy-1.0.0
rootdir: /app
collected 3 items

tests/test_article.py .                                                  [ 33%]
tests/test_web_scraper.py .                                              [ 66%]
tests/test_data/test_newsreadercli.py .                                  [100%]

=============================== warnings summary ===============================
../usr/local/lib/python3.12/site-packages/certifi/core.py:36
  /usr/local/lib/python3.12/site-packages/certifi/core.py:36: DeprecationWarning: path is deprecated. Use files() instead. Refer to https://importlib-resources.readthedocs.io/en/latest/using.html#migrating-from-legacy for migration advice.
    _CACERT_CTX = get_path("certifi", "cacert.pem")

-- Docs: https://docs.pytest.org/en/stable/warnings.html
========================= 3 passed, 1 warning in 0.65s =========================

// Enter inside docker container
> docker images
REPOSITORY            TAG       IMAGE ID       CREATED         SIZE
newsreader-cli-test   latest    2376cae71f9e   7 minutes ago   734MB

> docker run -it --entrypoint /bin/bash newsreader-cli-test

// Check contents of /app folder
root@0e8f62096f71:/# ls /app
 Dockerfile  LICENSE   README.md   docker-image   newsreadercli   requirements.txt   run_tests.sh   tests

// Check location of run_tests.sh
root@0e8f62096f71:/# ls run_tests.sh
run_tests.sh
root@0e8f62096f71:/# pwd
/
root@0e8f62096f71:/#