Create a container image with the buidler, so we can avoid copying the builder binary into the env container during the env container build.
Gather both stdout and stderr of a build into the logs.
* Add Dockerfile for builder + update release scripts
* Improve logging of buildermgr and builder
* Add support for environment-scoped buildcmd
* Fix fork/exec wd when package is a file
* Update tests to also include builder image
* Add logging to builder's fork/exec
