Merge pull request #2862 from yep/faster-git-clone

containers/docker: use shallow clone of single branch

containers/docker/develop-alpine/Dockerfile

 FROM alpine:3.4
 
 RUN \
-  apk add --update go git make gcc musl-dev         && \
-  git clone https://github.com/ethereum/go-ethereum && \
-  (cd go-ethereum && git checkout develop)          && \
-  (cd go-ethereum && make geth)                     && \
-  cp go-ethereum/build/bin/geth /geth               && \
-  apk del go git make gcc musl-dev                  && \
+  apk add --update go git make gcc musl-dev && \
+  git clone --depth 1 --branch develop https://github.com/ethereum/go-ethereum && \
+  (cd go-ethereum && make geth) && \
+  cp go-ethereum/build/bin/geth /geth && \
+  apk del go git make gcc musl-dev && \
   rm -rf /go-ethereum && rm -rf /var/cache/apk/*
 
 EXPOSE 8545

containers/docker/master-alpine/Dockerfile

 FROM alpine:3.4
 
 RUN \
-  apk add --update go git make gcc musl-dev         && \
-  git clone https://github.com/ethereum/go-ethereum && \
-  (cd go-ethereum && make geth)                     && \
-  cp go-ethereum/build/bin/geth /geth               && \
-  apk del go git make gcc musl-dev                  && \
+  apk add --update go git make gcc musl-dev && \
+  git clone --depth 1 https://github.com/ethereum/go-ethereum && \
+  (cd go-ethereum && make geth) && \
+  cp go-ethereum/build/bin/geth /geth && \
+  apk del go git make gcc musl-dev && \
   rm -rf /go-ethereum && rm -rf /var/cache/apk/*
 
 EXPOSE 8545