drm_hwcomposer: CI: use local container image for building

The freedesktop instance provides a docker container registry with
ubuntu 23.04 based images ready to be used for building drm_hwcomposer.

Migrate to using that container image in order to:
- Avoid apt-get installing all dependencies for each job
- Remove duplication between the Dockerfile and the .gitlab-ci.yml

Since the container image provides aospless, we no longer need to download it
for each pipeline stage.
Also, this updates the folder structure:

Before:
/builds/drm-hwcomposer/drm-hwcomposer/
/builds/drm-hwcomposer/aospless

After:
/builds/drm-hwcomposer/drm-hwcomposer/
/home/user/aospless

For "tidy", this new structure requires to override the BASE_DIR variable.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
diff --git a/.ci/Makefile b/.ci/Makefile
index a30414c..3ca7560 100644
--- a/.ci/Makefile
+++ b/.ci/Makefile
@@ -1,5 +1,5 @@
 
-BASE_DIR:=../aospless
+BASE_DIR ?=../aospless
 
 SYSTEM_INCLUDE_DIRS := /usr/include/libdrm