build: adjust wrong path of zrepl source dir in build.Dockerfile

was symlinking /zrepl to /go/src/github.com/zrepl/zrepl earlier, forgot
to change that apparently

see 47726ad877

refs #38

build.Dockerfile

@@ -9,7 +9,7 @@ ENV ZREPL_LAZY_DOCS_REQPATH=/tmp/requirements.txt
 RUN /tmp/lazy.sh devsetup
 
 # prepare volume mount of git checkout to /zrepl
-RUN mkdir -p /go/src/github.com/zrepl
+RUN mkdir -p /go/src/github.com/zrepl/zrepl
 RUN chmod -R 0777 /go
 WORKDIR /go/src/github.com/zrepl/zrepl