docker: Use a volume for backup configuration

Otherwise we need to rebuild the image. Thanks @duez.
See !877
7 jobs for feature-s/backup-container in 10 minutes (queued for 2 seconds)
Name Stage Failure
failed
code_quality Test Quality
$ function propagate_env_vars() { # collapsed multi-line command
$ docker pull --quiet "$CODE_QUALITY_IMAGE"
unknown flag: --quiet
See 'docker pull --help'.
Uploading artifacts...
Runtime platform arch=386 os=linux pid=28690 revision=58272c27 version=12.7.0
WARNING: gl-code-quality-report.json: no matching files
ERROR: No files to upload
ERROR: Job failed: exit status 1