docker: make possible to only build the engine without committing it to Unvanquished

[illwieckz]

One can only build the engine by doing:

./docker-build --engine-reference=<git_reference> --targets=<engine_target>

The reference for the game is now optional, the current master branch will be built if not set. The currently committed engine reference for the game reference will be built if the engine reference is not set.

The repository clone is now complete, instead of doing a shadow clone. This makes easier to checkout arbitrary commits, and in the future this will ensure the most recent tag is reachable, to compute the version string.

[illwieckz]

For example I can build an arm64 static engine binary from this branch:

• Do explicit uint casting in GLSL operations DaemonEngine/Daemon#1613

This way:

./docker-build --engine-reference=illwieckz/unpack --targets=linux-arm64

Meaning the only dependencies I need on the device to test the build is Xorg and Mesa.

With this change I don't need anymore to commit the engine submodule in some temporary Unvanquished branch to be able to build the engine in docker.

[slipher]

LGTM to nuking DAEMON_DIR from build-release

I guess the other files are your personal scripts that don't need review.

[illwieckz]

Actually this proves to be very practical to me as it allowed me yesterday to bisect an engine build error that was only happening when building within docker. To proceed I opened two terminal, the one on the left with those docker scripts, one on the right with the dæmon repository, on a single branch pushed to upstream (here, master).

On every bisect step, I run the script on the left terminal using the reference given by the bisect step on the right terminal.

Without those changes, I would have to commit those engine reference for every bisect step into temporary branch/tag and push them to Unvanquished repository, and this would have been vert painful and noisy.

So I now consider this code battle tested.