Try docker image instead

It's very strange but on gitlab.com I managed to build a project with
the same set-up as the commented lines here. It might have to do with
the custom docker executor, but I have no idea why.

Let's try a docker image that seemed to work.

Might be related: https://github.com/godotengine/godot/issues/16949
1 job for master in 38 seconds
Status Job ID Name Coverage
  Build
failed #10695
librem5
purism

00:00:38

 
Name Stage Failure
failed
purism Build
ERROR: _fs_changed: Unknown export preset: Linux/X11
At: editor/editor_node.cpp:450.
EditorSettings Save OK!
ERROR: ~List: Condition ' _first != __null ' is true.
At: core/self_list.h:100.
WARNING: cleanup: ObjectDB Instances still exist!
At: core/object.cpp:1989.
/bin/bash: line 69: 11 Segmentation fault (core dumped) godot -v -q --path ./game --export "Linux/X11" ../build/$BUILD_NAME.x64
ERROR: Job failed: exit code 1