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 #387
docker-alpine x86_64
build-x86_64

00:00:14

failed #382
docker-alpine x86_64
build-x86_64

00:00:21

failed #381
docker-alpine x86_64
build-x86_64

00:00:17

failed #380
x86_64 docker-alpine
build-x86_64

00:00:26

failed #375
x86_64 docker-alpine
build-x86_64

00:00:22

 
  Upload
skipped #376
s390x docker-alpine
upload-s390x
skipped #377
x86_64 docker-alpine
upload-x86_64
 
  Manifest
skipped #378
docker-alpine
create-manifest
 
Name Stage Failure
failed
build-x86_64 Build
OK: 6 MiB in 14 packages
fetch http://dl-cdn.alpinelinux.org/alpine/v3.10/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.10/community/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/edge/testing/x86_64/APKINDEX.tar.gz
ERROR: unsatisfiable constraints:
gitlab-runner (missing):
required by: world[gitlab-runner]
The command '/bin/sh -c echo '@edge/testing http://dl-cdn.alpinelinux.org/alpine/edge/testing' >> /etc/apk/repositories && apk upgrade -U --no-cache && apk add -U --no-cache gitlab-runner@edge/testing' returned a non-zero code: 1
ERROR: Job failed: exit code 1