dockerfile: don't add gitlab-runner user

The gitlab-runner alpine package now creates the user, so we no longer
need to do this in the Dockerfile.
10 jobs for master in 2 minutes and 22 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Build
passed #374
docker-alpine s390x
build-s390x

00:02:08

failed #388
docker-alpine x86_64
build-x86_64

00:00:13

failed #375
docker-alpine x86_64
build-x86_64

00:00:22

failed #380
docker-alpine x86_64
build-x86_64

00:00:26

failed #381
docker-alpine x86_64
build-x86_64

00:00:17

failed #382
docker-alpine x86_64
build-x86_64

00:00:21

failed #387
docker-alpine x86_64
build-x86_64

00:00:14

 
  Upload
skipped #376
docker-alpine s390x
upload-s390x
skipped #377
docker-alpine x86_64
upload-x86_64
 
  Manifest
skipped #378
docker-alpine
create-manifest
 
Name Stage Failure
failed
build-x86_64 Build
No job log