First-stage CI structures

[cissieAB]

• build-containers.yml: build docker containers. At first stage, let's trigger it by pushing to a certain branch. Ask epsci macOS server to do it at backend, so I do not care the RHEL-related failures.
  There were successful builds on my mac while epscimac docker is down.

• Cache the built docker images.
  See the actions in my fork: https://github.com/cissieAB/phasm/actions/caches

• Write a testing.yml for verification and validation. Use the cached docker images.

  • basic_env_baisc_tests: unit tests, mag_field example
  • julia_env_julia_tests: julia tests
  • (later) gpu runner: gpu examples, leveraging ifarm self-runner
  • (later) rest tests