Embed image name in build

[JoshKarpel]

For HTMap to work as expected in all cases, it should use the image it is being run from as the execute image. Unfortunately, this information (org/repo:tag) is not available from inside the image, so we need to embed it into the image at build-time as an environment variable. This turns out to be non-trivial and build-platform-specific; will need to think about the right way to do this.

Note: this will not help anyone who FROMs this image... so we may need a better option. Or we just throw our hands up...