use FALLBACK_IMAGE when no devcontainer specified

[bpmct]

We currently have FALLBACK_IMAGE. If an devcontainer is not specified, we should also support a KITCHENSINK_IMAGE (or use the fallback image for this too). Then, an admin can maintain an image that works in "most" cases.

[michaelbrewer]

I do like this as a generic option to support "Open in Coder" for all projects and would enable a browser extension for Coder (see coder/coder#8589)

[bpmct]

We have several customers heavily using the kitchensink image approach, I think we should invest in this so that an envbuilder template can be a generic template.

Open question: is fallback image good enough, or is there a rationale to have both?

[michaelbrewer]

Fall back would suite my needs. I still need to see if I can get private nexus authentication to work.

[johnstcn]

Given: a user runs envbuilder with the following:

• Git repo that does not contain a devcontainer spec
• FALLBACK_IMAGE=some_image:some_tag

When: envbuilder is run

Then: envbuilder should gracefully continue without a devcontainer and build an environment consisting of FALLBACK_IMAGE.

[bpmct]

Totally makes sense!