[dashboard] Don't show 'Continue with Default Image' button while the build is still running

[jankeromnes]

Fixes #3777

While going down the rabbit hole of trying to fix the Continue with Default Image button while a build is still running in #3993, we realized that running Docker builds cannot be interrupted. image-builder simply wasn't designed for that.

So we have 3 choices:

1. Change image-builder so that it can interrupt builds that are currently in progress

2. Just start a new workspace instance while the old instance is still building, and live with having two instances in this case (breaks many assumptions in the code)

3. Remove the Continue with Default Image button while the build is still running, for now (this was also the case in the previous dashboard design -- button only showed up when the build finished / failed)

This PR implements choice 3.

How to test:

• Log into https://jx-dont-continue-with-default-img.staging.gitpod-dev.com/login
• Open https://jx-dont-continue-with-default-img.staging.gitpod-dev.com/#https://github.com/jankeromnes/gitpod-hanging-build and verify that there is no Continue with Default Image button (Docker build will hang for 1h)
• Open https://jx-dont-continue-with-default-img.staging.gitpod-dev.com/#https://github.com/jankeromnes/gitpod-build-error and verify that there is a Continue with Default Image button and that it works (Docker build will quickly fail)

[AlexTugarev]

nit: the disablement is quite hidden now. would extracting to a dedicated handler as a local help to make it more readable?

[AlexTugarev]

Works as advertised!

[jankeromnes]

🎰

/werft run

👍 started the job as gitpod-build-jx-dont-continue-with-default-img.4

[gtsiolis]

FWIW, Tried this earlier and worked as expected! 🎰