CI for image building

closes #16 (closed)

Add CI for building images.

Different ways of building images:

1. Changes pushed to cms/cvmfs folders
   • Trigger build of cms_cvmfs template
2. Schedule including the variable IMAGE_NAME
   • Trigger build of cms_cvmfs template
3. (e.g.) API call including variables RELEASE, SCRAM_ARCH and OS
   • Trigger build of standalone template

Changes:

• Use build template for both cms_cvfms and standalone.
• Trigger check_changed_files only when new changes are pushed (avoid triggering on API/pipeline)