fix(CI): Use cubicweb/dockerfiles images to run py3 tests

The is not available on open-source project
5 jobs for !4 with topic/default/fix-ci in 2 minutes and 29 seconds (queued for 4 seconds)
merge request
Name Stage Failure
py3 Tests
Uploading artifacts for failed job
Uploading artifacts...
WARNING: py3-deprecated-warnings.json: no matching files
ERROR: No files to upload
Uploading artifacts...
report.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=430491 responseStatus=201 Created token=q9ySCzf4
Cleaning up file based variables
ERROR: Job failed: command terminated with exit code 1