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
This change orchestrates function builds.
Environments (in v2) define a builder image, just like they do a runtime image. The builder image contains a build script that's invoked with source and deployment paths (as env vars).
The buildermgr watches for environments with build images defined, and creates build deployments and services.
Functions can define source and deployment. Buildermgr watches for functions with source code (and build status == pending) and invokes the environment's builder when appropriate. It captures logs from the build and sets the build status (success/failure) and build lots into the PackageStatus.
Build server and client for environments.
This build server will run inside environment builders and invoke environment-specific scripts for doing builds. It provides a uniform language-independent API for builds to the fission build manager.
The change also includes an implementation of a builder for Python -- this "builder" acts on requirements.txt and runs `pip install` to collect all function deps.
