Normalize Docker image name for building a static image via matrix

What does this MR do and why?

Normalize Docker image name for building a static image via matrix. Normalizing the image name unlocks the ability for using parallel:matrix keyword in out CI pipeline for building multiple images at once, for instance, aarch64 and amd64. Before we used bash argument $1 for getting the image name but in parallel:matrix builds this argument has a colon : at the end which is invalid for a Docker image name. The fix is quite simple - remove the colon before starting the build.

parallel:matrix enabled?	$1 value
No	danger
Yes	danger:

See it in action here - https://gitlab.com/gitlab-org/gitlab-build-images/-/pipelines/970662467

Checklist

• (If applicable) Add patches if required for upgrading Ruby version in https://gitlab.com/gitlab-org/gitlab-build-images/-/tree/master/patches/ruby.