Do not stretch background image

Fixes #1
3 jobs for master in 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
failed #77393
build

00:00:57

 
  Test
skipped #77395
allowed to fail
code_quality
skipped #77394
test
 
Name Stage Failure
failed
build Build
Skipping Git submodules setup
$ if [[ -z "$CI_COMMIT_TAG" ]]; then # collapsed multi-line command
$ /build/build.sh
Building Heroku-based application using gliderlabs/herokuish docker image...
invalid reference format
invalid reference format
invalid argument "/master:8ec60f3f583bc7f4db8d25c9a5df08eb843949ae" for "-t, --tag" flag: invalid reference format
See 'docker build --help'.
ERROR: Job failed: exit code 1