Container build fixes + docker-compose

[ebr]

[WIP] this requires #3547 to be merged for the web UI to work correctly. PR is draft until then

The PR can be tested locally by merging it with #3547 and running docker-compose up --build (see README for details)

Changes in this PR:

• Replace Bash build scripts with docker-compose
• Rewrite Dockerfile:
  • enables running arbitrary commands (such as invokeai-configure)
  • adds a build stage to build the UI, so that it's always up to date in the image
  • adds magic-wormhole and SSH support, useful for Runpod etc
  • no longer uses Docker volumes by default (user may still choose to do so via docker-compose.yml
  • an existing v3.0.0-compatible runtime directory can be mounted and used interchangeably with a native (non-docker) install
  • run as a non-privileged user
• Additional README and docs used in the Runpod template

Still TODO:

• update CI workflows for container building