Update spack stack create ctr: make specs configurable, remove warnings, add CI builds

[climbfuji]

Summary

Changes for how we create containers, and CI testing for them.

1. With Update spack stack extension for containers: make specs configurable, clean up arguments for env/ctr spack#333, the spack stack create ctr command is updated, it takes four options (--container, --specs, optional --dir, optional --overwrite). This way, we don't need to manually add specs to the container recipes files, and we also get rid of annoying warnings from invalid options in the container spack.yaml.
2. Updated the documentation with these changes.
3. Created a CI test for container builds, because this takes so long I decided to run them on a schedule (this is UTC time, there are already nightly scheduled github actions at 2 UTC and 4 UTC for other JCSDA projects), a different container build / spec combination each day (only one spec so far --> just rotate the containers). CI can also be triggered for PRs via labels (but no choice of container or spec) or via workflow_dispatch (can choose spec, container). Because of the no-choice limitation for containers/specs for PR builds, I am not sure how valuable those are. Maybe workflow_dispatch is good enough? Keep it simple.

Testing

• Manually built containers on the Ubuntu CI instance first
• CI build (using docker-ubuntu-gnu-openmpi with jedi-ci)

Applications affected

JEDI CI container builds

Systems affected

None

Dependencies

• waiting on Update spack stack extension for containers: make specs configurable, clean up arguments for env/ctr spack#333

Issue(s) addressed

Resolves #146

Checklist

• This PR addresses one issue/problem/enhancement, or has a very good reason for not doing so.
• These changes have been tested on the affected systems and applications.
• All dependency PRs/issues have been resolved and this PR can be merged.

[climbfuji]

An actual bug fix, this should not have been updated when spack-stack-1.5.0 was merged into develop.