Merge branch 'update-to-alpine-3.8' into 'master'

Update docker images to alpine:3.8

Closes #3418

See merge request !984

dockerfiles/alpine/Dockerfile

-FROM alpine:3.7
+FROM alpine:3.8
 
 RUN adduser -D -S -h /home/gitlab-runner gitlab-runner
 

dockerfiles/build/Dockerfile.arm

-FROM multiarch/alpine:armhf-v3.7
+FROM multiarch/alpine:armhf-v3.8
 
-RUN apk add --update bash ca-certificates git miniperl \
+RUN apk add --no-cache bash ca-certificates git git-lfs miniperl \
 	&& ln -s miniperl /usr/bin/perl
 
+RUN git lfs install
+
 COPY ./scripts/ /usr/bin
 COPY ./binaries/gitlab-runner-helper.arm /usr/bin/gitlab-runner-helper
 

dockerfiles/build/Dockerfile.x86_64

-FROM alpine:3.7
+FROM alpine:3.8
 
 RUN apk add --no-cache bash ca-certificates git git-lfs miniperl \
 	&& ln -s miniperl /usr/bin/perl