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
Add a flag to the environment to control multiple specialization -- the max number of functions per container. This can be set to 1 or infinity.
Add an api proxy to workflow apiserver from the controller.
Add function metadata to FunctionLoadRequest; every v2 environment now knows which function it's loading (but can ignore that information if it wants to).
Add function identity headers to router. This is useful for multiple specialization, so the router can disambiguate between different function calls. (If this turns out to be a non-trivial perf overhead, we could add these headers conditionally, but for now they are always added.)
