Package and distribute dependencies of UBI-based CNG images for offline build

This MR is part of the solution for packaging and distributing dependencies of UBI-based CNG images for offline build.

Semantically, it contains two sets of changes.

The first set of changes:

• Factors out the build stage of Dockerfile.ubi8 into Dockerfile.build.ubi8 files,
• Introduces new build:X jobs and adds them to phase-N:prepare stages (which finish before X job and phase-N stage). These jobs build images from Dockerfile.build.ubi8 files and copy the binary assets from them (located at /assets directory inside the image) to artifacts/ubi/X in build artifacts (using copy_assets function).
• When ${UBI_PIPELINE} = 'true' the required assets from build artifacts are copied to the build context of the final image (using use_assets function). Dockerfile.ubi8 files are changed to copy anything that is included in /assets to their file system root. Dockerfile.build.ubi8 files preserve the directory structure of binary assets.

NOTE: .gitlab-ci.yml contains a few flags to short circuit the pipeline for test purposes. They will be removed once the tests are done.

The second set of changes is still WIP and contains code to package, sign, and release the UBI dependencies.

Closes gitlab-org/distribution/team-tasks#503 (closed) and gitlab-org/distribution/team-tasks#474 (closed)