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

environments/python/builder/Dockerfile

@@ -1,4 +1,5 @@
-FROM alpine:3.5
+ARG BUILDER_IMAGE=fission/builder:latest
+FROM ${BUILDER_IMAGE}
 
 RUN apk update
 RUN apk add --no-cache python3 python3-dev build-base
@@ -6,6 +7,5 @@ RUN pip3 install --upgrade pip
 RUN rm -r /root/.cache
 
 ADD defaultBuildCmd /usr/local/bin/build
-ADD builder /builder
 
 EXPOSE 8001