Dockerfile: Add git dependency to Dockerfile (#3295)

Dockerfile

@@ -2,10 +2,10 @@ FROM alpine:3.3
 
 ADD . /go-ethereum
 RUN \
-  apk add --update go make gcc musl-dev             && \
+  apk add --update git go make gcc musl-dev         && \
   (cd go-ethereum && make geth)                     && \
   cp go-ethereum/build/bin/geth /geth               && \
-  apk del go make gcc musl-dev                      && \
+  apk del git go make gcc musl-dev                  && \
   rm -rf /go-ethereum && rm -rf /var/cache/apk/*
 
 EXPOSE 8545