containers/docker: bump legacy images to 1.8 branch (#16084)

containers/docker/master-alpine/Dockerfile

@@ -2,7 +2,7 @@ FROM alpine:3.7
 
 RUN \
   apk add --update go git make gcc musl-dev linux-headers ca-certificates && \
-  git clone --depth 1 --branch release/1.7 https://github.com/ethereum/go-ethereum && \
+  git clone --depth 1 --branch release/1.8 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

@@ -5,7 +5,7 @@ ENV PATH=/usr/lib/go-1.9/bin:$PATH
 RUN \
   apt-get update && apt-get upgrade -q -y && \
   apt-get install -y --no-install-recommends golang-1.9 git make gcc libc-dev ca-certificates && \
-  git clone --depth 1 --branch release/1.7 https://github.com/ethereum/go-ethereum && \
+  git clone --depth 1 --branch release/1.8 https://github.com/ethereum/go-ethereum && \
   (cd go-ethereum && make geth) && \
   cp go-ethereum/build/bin/geth /geth && \
   apt-get remove -y golang-1.9 git make gcc libc-dev && apt autoremove -y && apt-get clean && \