Apply DSOP Dockerfile guidelines on UBI-based CNG images

This MR contains changes that are need to conform with DSOP Dockerfile Guidelines. For more details about these changes see gitlab-org/distribution/team-tasks#505 (closed).

Notes for reviewer(s):

• Every gitlab image that is used in Chart has a set of standard labels.
• All images except gitlab-task-runner have HEALTHCHECK (gitlab-task-runner is used as a shell).
• All images except gitlab-redis-ha explicitly use a non-root USER.
• gitlab-sidekiq uses GITLAB_VERSION for version and release labels.

Closes gitlab-org/distribution/team-tasks#505 (closed)