more reproducible build for rmf-web

[koonpeng]

This PR attempts to make the docker builds for rmf-web slightly more reproducible.

• Fetching of sources is done outside the build to make docker layer caching work. Also makes it easier to integrate with CI/CD as they usually have their own mechanism to fetch sources.
• Images are tagged with the commit sha of the sources.
• Use humble compatible commit, remove hacks to force it to build.
  • TODO: The commit is not merged yet, since we use squash merge, it will be gone when it is merged.
• Remove "hardcoded" rmf dashboard resources, a script is used to fetch the resources instead.
  • For the template, the script is just a dummy that echo some instructions. Users are expected to write their own script if they need custom dashboard resources.

Note that this does not make the build reproducible because it depends on the rmf build. rmf.repos used in the rmf build points to the main branch so every rmf build is potentially different.

[youliangtan]

I believe with the recent merge of #7, some features mentioned here is now in main. One thing which is missing in the current main:

Remove "hardcoded" rmf dashboard resources, a script is used to fetch the resources instead.