Fix registration permission denied

For normal operations, the `runner` user's `.gitlab-runner` directory
has a symlink for (read-only) `config.toml` ->
`/var/run/secrets/config.toml`.  However, for during registration
attempts to write to this volume-mounted file result in a
`PANIC: write: permission denied` error.  Fix this by running the
registration as (namespaced) `root` user, and using the default config.
file location of `/etc/gitlab-runner`.

Signed-off-by: Chris Evich <cevich@redhat.com>

Containerfile

@@ -43,7 +43,7 @@ ARG NESTED_PRIVILEGED=true
 # may be changed if you know what you're doing.
 LABEL register="podman run -it --rm \
  --secret=REGISTRATION_TOKEN,type=env \
- -v ./config.toml:/home/runner/.gitlab-runner/config.toml:Z \
+ -v ./config.toml:/etc/gitlab-runner/config.toml:Z \
  -e REGISTER_NON_INTERACTIVE=true \
  -e CI_SERVER_URL=https://gitlab.com/ \
  -e RUNNER_NAME=pipglr \
@@ -56,7 +56,6 @@ LABEL register="podman run -it --rm \
  -e DOCKER_VOLUMES=/cache \
  -e DOCKER_NETWORK_MODE=host \
  -e DOCKER_PRIVILEGED=${NESTED_PRIVILEGED} \
- --user runner \
  --entrypoint=/usr/bin/gitlab-runner \$IMAGE register"
 # Additionally, the nested-podman storage volumes must be pre-created with
 # 'podman' UID/GID values to allow nested containers access.