Optimize Dockerfiles for faster and smaller builds

[tdruez]

Current sizes:

• Uncompressed: scancodeio-images-v30.0.1.tar - 5.15 GB
• Compressed: scancodeio-images-v30.0.1.tar.gz - 1.76 GB

Some ideas at https://testdriven.io/blog/docker-best-practices/ and https://testdriven.io/blog/dockerizing-django-with-postgres-gunicorn-and-nginx/

[tdruez]

From 1.76 GB to 713M for the compressed tarball with current changes in the https://github.com/nexB/scancode.io/tree/350-docker-optimizations branch
Also, the build is much faster and leverage caching.

[tdruez]

Step 8/9 : RUN --mount=type=cache,target=/root/.cache/pip pip install .
ERROR: Service 'web' failed to build : the --mount option requires BuildKit. Refer to https://docs.docker.com/go/buildkit/ to learn how to build images with BuildKit enabled

The following requires BuildKit which is not always enabled depending on the Docker version

# Cache Python packages on the local machine
RUN --mount=type=cache,target=/root/.cache/pip pip install .