containers/docker: bump docker images to 1.7 release branch

containers/docker/master-alpine/Dockerfile

@@ -2,7 +2,7 @@ FROM alpine:3.5
 
 RUN \
   apk add --update go git make gcc musl-dev linux-headers ca-certificates && \
-  git clone --depth 1 --branch release/1.6 https://github.com/ethereum/go-ethereum && \
+  git clone --depth 1 --branch release/1.7 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 linux-headers && \

containers/docker/master-ubuntu/Dockerfile

@@ -3,7 +3,7 @@ FROM ubuntu:xenial
 RUN \
   apt-get update && apt-get upgrade -q -y && \
   apt-get install -y --no-install-recommends golang git make gcc libc-dev ca-certificates && \
-  git clone --depth 1 --branch release/1.5 https://github.com/ethereum/go-ethereum && \
+  git clone --depth 1 --branch release/1.7 https://github.com/ethereum/go-ethereum && \
   (cd go-ethereum && make geth) && \
   cp go-ethereum/build/bin/geth /geth && \
   apt-get remove -y golang git make gcc libc-dev && apt autoremove -y && apt-get clean && \