Set image labels based on repository info

[RaphvK]

Closes #64

Currently, all images created in the docker-ros workflow inherit the following image labels from the base image (when used with default docker-ros-ml-images):

{
"maintainer":"Institute for Automotive Engineering (ika), RWTH Aachen University <opensource@ika.rwth-aachen.de>",
"org.opencontainers.image.authors":"Institute for Automotive Engineering (ika), RWTH Aachen University <opensource@ika.rwth-aachen.de>",
"org.opencontainers.image.licenses":"MIT",
"org.opencontainers.image.url":"https://github.com/ika-rwth-aachen/docker-ros-ml-images",
"org.opencontainers.image.version":"26.04"
}

This PR extends the build scripts and Dockerfile so that the following labels are overridden:

• "maintainer": overridden with empty string by default
• "org.opencontainers.image.authors": overridden with empty string by default
• "org.opencontainers.image.licenses": taken from LICENSE file in root of repository if present, empty string otherwise
• "org.opencontainers.image.url": URL of git repository if the build command is run from a git repo, empty string otherwise
• "org.opencontainers.image.version": Git commit hash or tag if run in a git repo or tag pipeline, empty string otherwise

All values can be overriden using CI environment variables.

Tests

• Local build in git repo
• Local build in folder that is not in git repo
• GitHub CI pipline (openads_ros2_demo_repository)
• GitLab CI pipeline (point_cloud_fusion)

[RaphvK]

As discussed with @lreiher and @jpbusch:

• get url and version from repo metainfo
• do not parse package.xml as this might lead to errors.
• get license from LICENSE file in repo root
• allow setting maintainer and authors via ci environment variables, unset as default
• add one extra label to identify image as docker-ros image: "de.rwth-aachen.ika.docker-ros": "true"

[RaphvK]

As discussed with @lreiher and @jpbusch:

* get url and version from repo metainfo

* do not parse `package.xml` as this might lead to errors.

* get license from `LICENSE` file in repo root

* allow setting maintainer and authors via ci environment variables, unset as default

* add one extra label to identify image as docker-ros image: `"de.rwth-aachen.ika.docker-ros": "true"`

Changes implemented as discussed and ready for review @lreiher @jpbusch