Docker-based build process

[jacoscaz]

This PR automates the management of the environment required for building the website using Docker. It is an alternative to the work started by @Tieske in #12 with the following advantages:

1. It builds the website using the same Docker-based environment both locally and in GitHub CI.
2. It uses a simpler approach to dependency management, with Hugo and Git implicitly pinned to the respective versions distributed in the explicitly pinned version of Alpine Linux.
3. It uses a simpler Dockerfile that doesn't require a dedicated entrypoint script.
4. It produces a significantly smaller Docker image (~ 48 MB).

Supersedes #12 .

The only thing I have not been able to test so far is the CI workflow. Docker is installed by default in the Ubuntu 22.04 runner so this should work OOTB. EDIT: seems to work fine now that this PR is open and CI is running.

@schaze does this allow you to test build the website locally? It should be as simple as running ./build.sh (assuming you have Docker installed)..

[Tieske]

works for me

[Tieske]

merged this one first. If the build succeeds, I'll merge #35