freeze Go build dependencies in Gopkg.lock

* use pseudo-depdencies in build/build.go to convince dep
* update Travis, Dockerfile and Docs
* build.Dockerfile image now contains the Go build dependencies
* => faster builds
* bump pdu file after protoc update

fixes #106

build.Dockerfile

@@ -11,12 +11,21 @@ RUN unzip -d /usr protoc-3.6.1-linux-x86_64.zip
 ADD lazy.sh /tmp/lazy.sh
 ADD docs/requirements.txt /tmp/requirements.txt
 ENV ZREPL_LAZY_DOCS_REQPATH=/tmp/requirements.txt
-RUN /tmp/lazy.sh devsetup
+RUN /tmp/lazy.sh docdep
 
 # prepare volume mount of git checkout to /zrepl
 RUN mkdir -p /go/src/github.com/zrepl/zrepl
-RUN chmod -R 0777 /go
 RUN mkdir -p /.cache && chmod -R 0777 /.cache
 WORKDIR /go/src/github.com/zrepl/zrepl
 
+ADD Gopkg.toml Gopkg.lock  ./
+
+# godep will install the Go dependencies to vendor in order to then build and install
+# build dependencies like stringer to $GOPATH/bin.
+# However, since users volume-mount their Git checkout into /go/src/github.com/zrepl/zrepl
+# the vendor directory will be empty at build time, allowing them to experiment with
+# new checkouts, etc.
+# Thus, we only use the vendored deps for building dependencies.
+RUN /tmp/lazy.sh godep
+RUN chmod -R 0777 /go