#2448: ci: bake: enable local output and use it for caching

[cz4rs]

fixes #2448

This is pretty simple and straightforward approach, with the main downside being that local output means exporting entire container (while we are only interested in /build/ccache). Using docker cp would probably achieve the same result with an additional step in build-docker-images.yml.

Other possibilities:

• look for the cache location for type=cache mounts: https://stackoverflow.com/a/79489859
• copy files from container to host: https://stackoverflow.com/a/51186557

[github-actions]

clang-format output for this changeset:

no modified files to format

[cz4rs]

ccache does not work correctly in Alpine build (even though the output from previous build is correctly stored / restored with actions/cache@v4):

#8 [build 2/3] RUN --mount=target=/vt <<EOF (if [ -d vt/docker-output/build/ccache ]; then...)
#8 0.159 Summary:
#8 0.159   Hits:               0 /    0
#8 0.159     Direct:           0 /    0
#8 0.159     Preprocessed:     0 /    0
#8 0.159   Misses:             0
#8 0.159     Direct:           0
#8 0.159     Preprocessed:     0
#8 0.159 Primary storage:
#8 0.159   Hits:               0 /    0
#8 0.159   Misses:             0
#8 0.159   Cache size (GB): 0.00 / 0.70 (0.00 %)
#8 0.159 
#8 0.159 Use the -v/--verbose option for more details.
#8 DONE 2.0s

Can be fixed in a separate issue.

[lifflander]

This looks great for CI. Good job finding a solution for that!