Merge pull request #2165 from karalabe/gmp-dep-master-alpine

containers/docker/master-alpine: add the legacy gmp lib back for now

containers/docker/master-alpine/Dockerfile

 FROM alpine:3.3
 
 RUN \
-  apk add --update go git make gcc musl-dev gmp-dev && \
+  apk add --update go git make gcc musl-dev gmp-dev gmp && \
   git clone https://github.com/ethereum/go-ethereum     && \
   (cd go-ethereum && make geth)                         && \
   cp go-ethereum/build/bin/geth /geth                   && \