Building Docker containers¶

We use GitLab CI/CD, to create our Python Docker containers.

First, we define the Python version in our pyproject.toml file of the project:
```
[project]
name = "my-app"
requires-python = "==3.12.*"
```

We then extract this string in our .gitlab-ci.yml file and pass it as a build argument to docker build:

build:
  stage: build
  only: [main]
  script:
    - export PY=$(sed -nE 's/^requires-python = "==(3\.[0-9]+)\.*"$/python\1/p' pyproject.toml)
    - >
      docker build
      --build-arg PY=$PY

Finally, we can use the extracted version in the Dockerfile to create a virtual environment in the build phase and install the Python version in the application stage:

FROM your-docker/build-image as build

ARG PY
RUN --mount=type=cache,target=/root/.cache \
    set -ex \
    && virtualenv --python $PY /app
…
FROM your-docker/app-image
…
RUN set -ex \
    && apt-get update -qy \
    && apt-get install -qy \
        -o APT::Install-Recommends=false \
        -o APT::Install-Suggests=false \
…
COPY --from=build --chown=app /app /app