Remove deprecated Container Scanning variables with Docker references

Deprecation Announcement

All Container Scanning variables that are prefixed by DOCKER_ in variable name are deprecated. This includes the DOCKER_IMAGE, DOCKER_PASSWORD, DOCKER_USER, and DOCKERFILE_PATH variables. Support for these variables will be removed in the GitLab 16.0 release. Use the new variable names CS_IMAGE, CS_REGISTRY_PASSWORD, CS_REGISTRY_USER, and CS_DOCKERFILE_PATH in place of the deprecated names.

As the DOCKER_{IMAGE,USER,...} variables were deprecated (#357264 (closed)), we now need to remove them.

See:

• !96172 (merged)
• gitlab-org/security-products/analyzers/container-scanning!2806 (merged)

Implementation plan

• backend Remove support for DOCKER_{IMAGE,USER,...} variables in container-scanning analyzer
• documentation Remove information about supported DOCKER_{IMAGE,USER,...} variables

Verification steps

• Create a new pipeline which references the registry.gitlab.com/security-products/container-scanning:6 image and confirm that the CS_IMAGE var works, while the DOCKER_IMAGE var no longer works.

  See Verification steps for more details.