Enable building of images outside of GitPod!

[azanar]

Description

Enable building of images outside of GitPod!

Related Issue(s)

Fixes #1037

How to test

Run: ```
docker build -t gitpod-io/workspace-images:latest .
docker-compose run gitpod-io/workspace-images:latest ./build-all.sh

Watch images build into the registry running within docker-compose!

## Release Notes
<!--
  Add entries for the CHANGELOG.md or "NONE" if there aren't any user facing changes.
  Each line becomes a separate entry.
  Format: [!<optional for breaking>] <description>
  Example: !basic auth is no longer supported
  See https://www.notion.so/gitpod/Release-Notes-513a74fdd23b4cb1b3b3aefb1d34a3e0
-->
```release-note
Workspace image creation no longer depends upon execution within GitPod. Execution outside of GitPod will require `docker` and `docker-compose`, or their `podman` counterparts.

Documentation

Will add documentation update if PR gets closed to landing. This will require a different incantation to get the image builds going.

[gitpod-io]