Generate the Gitpod Docker image regularly

From: #1108 (comment 558067215)

We have decided to go with this first iteration (left out the registry prefix registry.gitlab.com/gitlab-org/gitlab-development-kit/ for brevity:

• Create a stable image called gitpod-workspace:stable which we are going to utilize in gitlab-org/gitlab per default: gitlab!60029 (merged)
• Create support for a scheduled pipeline which has two additional jobs:
  • build gitpod-workspace:main around 0:00 UTC
  • manual job that pulls gitpod-workspace:main and pushes it again under the gitpod-workspace:stable
  • MR here: !1583 (merged)
• Enable scheduled job: https://gitlab.com/gitlab-org/gitlab-development-kit/-/pipeline_schedules/118797/edit after merging !1583 (merged), triggering it once manually to verify it works
• Document how one verifies and promotes new releases of gitpod-workspace:main- !1957 (comment 577570341).
• Create weekly issues to verify and promote the gitpod-workspace:main image.

Previous description

The following discussion from !1583 (merged) should be addressed:

• @ashmckenzie started a discussion: (+3 comments)

As building the image takes ~50 minutes, it could make sense to be very intentional about when to run it to avoid having each documentation change trigger it. When I started talking about this with @leipert, I think he mentioned https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/master/support/docker#L58 as one potential spot where this could be integrated. As alternative, @stanhu suggested to run a nightly build of this. I'm not entirely sure what the advantages and what the drawbacks would be for either solution, so I can't give my own recommendation, but I wanted to make sure to bring it up.

state of the image (2021-04-08)

Image size is over 5GB, the last update is on 2021-01-05 (3 months ago):