Depends on #197

As part of #128, we want to embed the envbuilder binary in container images that are pushed to a registry (see #213 for the push implementation).

This could be as simple as appending COPY /.envbuilder/bin/envbuilder /.envbuilder/bin/envbuilder to the Dockerfile. The binary could be located elsewhere too, so a lookup may be necessary.

This will embed an eventually stale envbuilder binary in the image, but we consider this OK as our recommendation is to lock the envbuilder version, updating it will result in a new build.

Acceptance Criteria:

1. Build an image using envbuilder with ENVBUILDER_CACHE_REPO=localhost:5000/envbuilder-cache or similar (after feat: push final image to cache repo #197 is merged)
2. Pull the resulting image
3. Validate that the same envbuilder binary is present in the same location in the resulting image