fission/builder image, log collection improvement in builder (#397)

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

types.go

@@ -151,7 +151,10 @@ type (
 		FunctionEndpointPort int32 `json:"functionendpointport"`
 	}
 	Builder struct {
-		Image   string `json:"image"`
+		// Image for containing the language runtime.
+		Image string `json:"image"`
+
+		// (Optional) Default build command to run for this build environment.
 		Command string `json:"command"`
 	}
 	EnvironmentSpec struct {