Fix ruby_docker image and implement easier tagging of images

• Earlier fix of ruby_docker lacked curl in the image, which failed the build. Added curl to the install list
• Existing method of tagging requires a git tag to be pushed, which is an extra effort. Instead we can use a VERSION file for tagging, similar to what we propose in its counterpart - omnibus-gitlab!1606 (merged) . Added such a file with this MR. We can possibly add an MR template also with a checkbox "Updated the image version" to make sure we don't miss it.

/cc @marin