feat(backport): Use non-root default user for Docker image

[matthewfeickert]

Description

• Backport PR feat: Use non-root default user for Docker image #2243
• Add non-root default user 'moby' with uid 1000 that owns the Python virtual environment.
  • Set default working directory to /home/moby/work/.
• Add .dockerignore for local builds.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Backport PR https://github.com/scikit-hep/pyhf/pull/2243
* Add non-root default user 'moby' with uid 1000 that owns the Python virtual environment.
   - Set default working directory to /home/moby/work/.
* Add .dockerignore for local builds.