Move docker dep commands to earlier in the build

This will let Docker cache the results of the vendor dependencies.
Making re-builds during testing faster.

Also clean-up spurious test & rm in ./configure

Dockerfile

@@ -7,11 +7,14 @@ RUN curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.s
 
 # Copy sources
 WORKDIR $GOPATH/src/github.com/pusher/oauth2_proxy
-COPY . .
 
 # Fetch dependencies
+COPY Gopkg.toml Gopkg.lock ./
 RUN dep ensure --vendor-only
 
+# Now pull in our code
+COPY . .
+
 # Build binary and make sure there is at least an empty key file.
 #  This is useful for GCP App Engine custom runtime builds, because
 #  you cannot use multiline variables in their app.yaml, so you have to