CI: use custom docker images for dependencies and GIMP builds
Basic idea:
- based of Debian Testing (the currently used image), create a a custom image for the debian-based builds and the win32 and win64 builds: three different images in total for now
- do no or only very minimal package installing in the individual build steps
Seems to work fine, and gets consistently lower build times than the builds where package installs happen all the time. Image expiry is set to 120 hours, if I interpret the parameters correctly.
This is the same approach as used by GEGL, has been copied from there and adjusted to use Debian Testing, just like the GIMP builds right now.
Edited by Bruno