Briefcase data directory is still being mounted in to containers for brutus user when root is used

[rmartin16]

Describe the bug

With #1331, the user in Docker containers can now be root as well as brutus; however, Briefcase always mounts the data directory in to .cache directory for brutus. This doesn't appear to be having adverse effects right now....but it should probably be reconciled.

Steps to reproduce

1. Using Docker Desktop, run briefcase open linux --target debian
2. Run whoami to see current user
3. Run ls -lh /home/brutus/.cache/

Expected behavior

I suppose it doesn't actually hurt anything to leave this as is....but it seems a lot more consistent if we use a user-agnostic location to bind mount the Briefcase data directory given we're not always using the brutus user now.

Screenshots

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Environment

• Operating System: pop os 22.04
• Python version: 3.10.12
• Software versions:
  • Briefcase: 0.3.16.dev20+g8b020d6c

Logs

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Additional context

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[freakboy3742]

@rmartin16 AFAICT, this should have been fixed by beeware/briefcase-linux-system-template#7 and beeware/briefcase-linux-appimage-template#34 - am I correct, or is there something else going on here? If it is fixed, do we need to add changenote-only PR to announce the bug fix?

[rmartin16]

This is a little different; here, I'm pointing out that we're still using /home/brutus/.cache/briefcase when the brutus user does not exist in the image; such as when using Docker Desktop on Linux and root is used in the image.

It's not an inherent problem....it's just a bit strange...especially without any context/history.

I think a more consistent approach would put the Briefcase data directory in root's home directory in this case or use a dedicated location in the filesystem like /briefcase.