Jordigh/update stable docker build

[jordigh]

I did it this way because the two builds aren't close enough to be a single one. It was easier to duplicate a little code than to create more abstractions.

My original commit messages follow:

• 6162de8
  workflows: add a dummy ext to the stable build

See a437dfa for details

• c4f06be
  workflows: use variable tags for the stable build

This will make it easier to do some testing while I make sure that
this build is correct.

• 059b2cd
  workflows: don't build stable twice, just push it twice

This now matches the docker_latest.yml setup. No point building
grist-ee/grist twice.

[paulfitz]

Looks plausible. Will approve on confirmation that the images built:

• Have no ext material in grist-oss.
• Shows an enterprise banner for grist-ee.
• Responds to "GRIST_ACTIVATION=dud-activation` environment variable in grist-ee and grist.