Manually triggering `#imagebuild` dramatically lower the time to open a workspace

Is your feature request related to a problem? Please describe

Not sure if this is a bug or a feature.
Loading a workspace that uses a custom docker image which is hosted in hub.docker.com, takes a long time.
Manually triggering #imagebuild dramatically lower the time it takes to open the workspace.

Describe the behaviour you'd like

This process should happen automatically, without waiting for the user to run it manually.

Describe alternatives you've considered

Additional context

The last branch where I was able to apply this technique:
https://gitpod.io/#imagebuild/https://github.com/rfay/ddev/tree/20211101_gitpod_experiments

I first opened the branch multiple times, and it took over a minute to open the workspace -
https://gitpod.io/#https://github.com/rfay/ddev/tree/20211101_gitpod_experiments

Then I ran the special #imagebuild trigger (which also took long time)

Afterward, every time I opened the workspace, it took much shorter time than before the #imagebuild trigger.
That speed improvement also improved openning a fork I created from that branch (using the same custom docker image)

The issue names the #imagebuild trigger and workspace-opening flow but no files or tests. Start by tracing how Gitpod handles #imagebuild for custom images from hub.docker.com, then compare that path with normal workspace startup. Done means the image build is initiated automatically and later workspace openings use the resulting speed improvement without manual triggering.