Common starter configuration files for including in all branches

This project has 8 branches, and each has a customizsed .gitlab-ci.yml file. However, many of the changes are repeated over several branches.

We could have all these changes in one file (stored in the 'main' branch say), and this file is included at the top, then any adjustments, eg different variable settings, can be made per branch. This will reduce the size of the 8 customized .gitlab-ci.yml files, reduce duplication and therefore make maintainance of the files easier.

Summary of Merge Requests for this issue

Branch	Lines added	Lines removed	Net change
main	+277	−66	+211
d9-basic	+4	-105	-101
d10-plus	+6	-143	-137
d10-profile	+3	-77	-74
d10-theme	+3	-102	-99
d11-recipe	+7	-121	-114
d7-basic	+2	-49	-47
d7-composer	+2	-55	-53

The overall change is 304 lines added, 718 deleted making a net reduction of 414 lines of duplication removed.