Merge branch 'update-docker-images-bases' into 'master'

Update docker images bases

See merge request gitlab-org/gitlab-runner!860

dockerfiles/alpine/Dockerfile

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

dockerfiles/ubuntu/Dockerfile

-FROM ubuntu:14.04
+FROM ubuntu:16.04
 
 RUN apt-get update -y && \
     apt-get upgrade -y && \