Fix #11, Add RTEMS 6 container for cFS test workflow

[ghost]

This pull request adds a RTEMS 6 container and appropriate workflow to build it and add it to the container repository. The RTEMS 6 container will be used to support the RTEMS 6 cFS build and tests.

[ghost]

Note that in my forked repository I was able to get all 3 containers to build correctly, but I could not push the completed container.

[astrogeco]

CCB:2022-05-18 APPROVED

• @acudmore open issue in this repo to remove network stack fix when RTEMS does the fix on their end
• choose a snapshot that works and open an issue to update it regularly
  • bonus points, automate with dependabot or a similar tool

[ghost]

I added a commit to use specific RTEMS repository commits, and documented the date for them in the Dockerfile.