Support custom environments in blueprint runner.

## Describe your feature request

I'd love to be able to 'pre-install' certain global dependencies like python, mvn, etc as part of the blueprint runner image itself.

## Describe the solution you'd like

Provide something like a devfile or dockerfile as part of the blueprint bundle which when invoked on the server during synth, will use the provided image as a base image to perform synth.

## Describe alternatives you've considered

- Install dependencies as part of synth via `exec` commands
- Bundle runtime deps as part of blueprint bundle (this becomes way too large)

## Additional context

I am trying to generate lockfiles for all supported languages/runtimes and as such need access to the runtimes at synth time so I can generate the lockfile. Pre-generating these for all permutations is not feasible and the only sure fire way is to generate them at synth time.

Start by tracing the blueprint runner's synth entry point and how the blueprint bundle is invoked; the issue names a devfile or Dockerfile as the proposed configuration. Compare the existing exec-based dependency installation and bundle runtime approach. Done means a supplied custom image can provide runtimes such as Python or Maven during synth without pre-generating every lockfile permutation.

Written by the indexing model from the issue text.