containers/docker: use shallow clone of single branch

containers/docker/develop-alpine/Dockerfile

@@ -2,8 +2,7 @@ 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)          && \
+  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                  && \

containers/docker/master-alpine/Dockerfile

@@ -2,7 +2,7 @@ FROM alpine:3.4
 
 RUN \
   apk add --update go git make gcc musl-dev         && \
-  git clone https://github.com/ethereum/go-ethereum && \
+  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                  && \