Dockerfile: use gitlab-runner from community

gitlab-runner has been moved to community, so we need to obtain it from
there. As we were getting gitlab-runner from edge anyway, switch the
image to use edge.
2 jobs for gitlab-runner-community in 36 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
failed #383
docker-alpine s390x
build-s390x

00:00:35

passed #384
docker-alpine x86_64
build-x86_64

00:00:23

 
Name Stage Failure
failed
build-s390x Build
Step 2/7 : ENV SSL_CERT_FILE /etc/ssl/cert.pem
---> Running in 953f8d071dc9
Removing intermediate container 953f8d071dc9
---> 87583efe3a20
Step 3/7 : RUN apk add -U --no-cache gitlab-runner
---> Running in 565b038b4a5f
standard_init_linux.go:211: exec user process caused "exec format error"
The command '/bin/sh -c apk add -U --no-cache gitlab-runner' returned a non-zero code: 1
ERROR: Job failed: exit code 1