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 && \
-  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 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                   && \
+  apk del go git make gcc musl-dev gmp-dev              && \
   rm -rf /go-ethereum && rm -rf /var/cache/apk/*
 
 EXPOSE 8545