Update Dockerfile

3 jobs for master in 14 seconds (queued for 2 seconds)
Name Stage Failure
build Build
WARNING! Your password will be stored unencrypted in /root/.docker/config.json.
Configure a credential helper to remove this warning. See

$ docker build --pull -t $TEST_IMAGE .
Sending build context to Docker daemon 78.85kB
Step 1/2 : FROM centos7
pull access denied for centos7, repository does not exist or may require 'docker login': denied: requested access to the resource is denied
ERROR: Job failed: exit code 1