chore: add java-shared-config to safe.directory and mark docker validation check as required

[mpeddada1]

The Kokoro - Test:Docker Image Validation is returning empty results for branchName and gitCommitMessage in #687. This is leading to the check passing for the incorrect reason (i.e the check passes even though gcr.io/cloud-devrel-public-resources/graalvma:1.6.0 has not been published.) Root cause from logs:

Executing: docker run --rm --interactive --network=host --privileged --volume=/var/run/docker.sock:/var/run/docker.sock --workdir=/tmpfs/src --entrypoint=github/java-shared-config/.kokoro/verify-docker-images.sh --env-file=/tmpfs/tmp/tmp80fng0kc/envfile --volume=/tmpfs:/tmpfs gcr.io/cloud-devrel-kokoro-resources/java8
fatal: detected dubious ownership in repository at '/tmpfs/src/github/java-shared-config'
To add an exception for this directory, call:

	git config --global --add safe.directory /tmpfs/src/github/java-shared-config
fatal: detected dubious ownership in repository at '/tmpfs/src/github/java-shared-config'
To add an exception for this directory, call:

	git config --global --add safe.directory /tmpfs/src/github/java-shared-config
fatal: detected dubious ownership in repository at '/tmpfs/src/github/java-shared-config'
To add an exception for this directory, call:

	git config --global --add safe.directory /tmpfs/src/github/java-shared-config

[mpeddada1]

Adding git config --global --add safe.directory ... is now returning non-empty results for git name-rev and git log:

Check if git is installed:
/usr/bin/git
2f181de04aba21c33e340d6cd834d8f3a6267406
remotes/origin/test-docker-check

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• .github/sync-repo-settings.yaml