Checkout the repository with git ...

This changes the build process even further. Instead
f using `wget` to fetch the current code, `git` is used.
This allows for faster switching between branches,
because only the differences between them have to be
fetched from the server.

But the main advantage is that the build cache can
finally be used as designed by Docker. Repetitive
builds are very fast now. This is also true between
branches and tags, as long as the `requirements.txt`
file doesn't change.

Refactor to multistage builds ...

This commit introduces a huge change in the build process.

What changed:

- Dockerfile.ldap was integrated into Dockerfile as a seperate
  [build stage][multistage-build].
- All the build scripts were refactored according to this.
- The `docker-compose.yml` file was adjusted likewise.
- The main build script, `/build.sh`, now always builds all
  targets (formerly called variants).
- The minimal requirements for Docker and docker-compose
  have increased.
- The build on hub.docker.com must be adjusted.

This change should also fix #156 permanently.

[multistage-build]: https://docs.docker.com/develop/develop-images/multistage-build/

Build system for hub.docker.com

⚙️ Don't build webhook branch anymore ...

Because it's merged into Netbox since 2.4.

Revert "⚙️ Print every command" ...

This reverts commit 1fb6b80.

⚙️ Print every command

⚙️ Additional build debug info

⚙️ Parallel builds

🐞 Fix error reporting

⚙️ Compact Variants Logic into build-all.sh