ci: Update all Debian CI runners to use v5 of the Dockerfile
ci: Update all Debian CI runners to use v5 of the Dockerfile

This shouldn’t affect them (since it just adds the `clang-format-7`
package), but it’s good to keep everything on the same version.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

See: #1552

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib