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

builder/cmd/Dockerfile

@@ -0,0 +1,9 @@
+FROM alpine:3.5
+
+RUN apk update
+# Install some utility libraries for builders extending image
+RUN apk add coreutils binutils findutils grep
+
+ADD builder /builder
+
+EXPOSE 8001