CI: Update image, not using base anymore
CI: Update image, not using base anymore

Since base is now 200MB smaller, this one is smaller too. The
fdroidserver-only deps are now in the client image - mainly python3-dev,
gcc and all the build deps for stuff like pillow.

f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver