Merge branch 'master' into 357-support-pdf-generation-via-electron-pdf-running-in-the-background-using-celery

Resolve "Update to Django 3.2"

Closes #421

See merge request !564

Update lockfile

See merge request !561

Resolve "[Docker] Review handling of volumes"

Closes #403

See merge request !543

This allows making the STATIC_ROOT read-only

Requires django-sass-processor 1.0

Resolve "[DOM] Found 2 elements with non-unique id #language-select"

Closes #418

See merge request !558

Resolve "404 pages broken due to django-menu-generator-ng"

Closes #411

See merge request !557

[Docker] Do not generate a secret key on startup

See merge request !556

This would require www-data to be able to write the secret key now.
That would not be an issue in general, but requires awkward changes
with no real use. Users are supposed to configure a secret key
themselves.

Update dependency calendarweek to ^0.5.0

See merge request !555

Merge branch '413-docker-allow-using-core-image-as-base-image-for-app-distribution-images' into 'master'

Resolve "[Docker] Allow using core image as base image for app/distribution images"

Closes #413

See merge request !550