use go 1.6.2 for docker go build

[chris-rock]

I switched to the official golang image. Not sure if there was a reason to use the dex-builder image?

supersedes #406
fixes #404

[ericchiang]

Never cared for the official Go image (it's like 750MB for a 80MB Go distro, ships with a python interpreter, etc), but it looks like the dex-builder is just a derivative of it anyway.

$ cat Dockerfile-builder
FROM golang:1.6

RUN go get golang.org/x/tools/cmd/cover
RUN go get golang.org/x/tools/cmd/vet
RUN go get github.com/tools/godep

Should be fine, we use vendor now, and I believe vet and cover are now built into the tool.

We should also remove Dockerfile-builder if we're going to delete this.

@bobbyrullo any objections?

[bobbyrullo]

Seems reasonable to me

[chris-rock]

Updated the PR to remove the Dockerfile-builder as well

[ericchiang]

LGTM. Thanks for the update!