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. |
| ENTRYPOINT |
This command specifies the default command that will run when the container starts. |
Dockerfile
What is Dockerfile?
From Dockerfile, docker image is created which runs inside container.
Example
Let's create a docker image.
We will use the code from this repository https://github.com/meobilivang/newsreader-cli
// 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.
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:/#