Ulimit deemed required by config validation

Summary

16.2 gitlab-runner added ulimit parameter for docker executor. When registering a runner with non-interactive one-line command, the ulimit field is not populated (and I think that's perfectly fine). After this, if you run gitlab-runner list, you get a validation warning about wrongly formatted (missing) ulimit.

Steps to reproduce

Register a runner with

sudo REGISTER_NON_INTERACTIVE=true gitlab-runner register --url https://gitlab.com --token "redacted" --executor "docker" --docker-image "alpine:3" --docker-privileged --docker-tlsverify --docker-volumes "/certs/client" --docker-volumes "/cache"

and then run

sudo gitlab-runner list

and you will get a validation warning:

Runtime platform                                    arch=amd64 os=linux pid=3731 revision=674e0e29 version=16.2.1
There might be a problem with your config based on jsonschema annotations in common/config.go (experimental feature):
jsonschema: '/runners/0/docker/ulimit' does not validate with https://gitlab.com/gitlab-org/gitlab-runner/common/config#/$ref/properties/runners/items/$ref/properties/docker/$ref/properties/ulimit/type: expected object, but got null

Actual behavior

Warns about incorrectly formatted ulimit

Expected behavior

Should have ulimit as optional in schema and validate without warning when ulimit is omitted.

Environment description

Running our own runner on an EC2 instance with gitlab-runner 16.2.1.

config.toml contents

listen_address = "0.0.0.0:9252"
concurrent = 6
check_interval = 0
shutdown_timeout = 0

[session_server]
  session_timeout = 1800

[[runners]]
  name = "ip-10-0-redacted.eu-central-1.compute.internal"
  url = "https://gitlab.com"
  id = 258redacted
  token = "glrt-redacted"
  token_obtained_at = 2023-08-15T05:37:18Z
  token_expires_at = 0001-01-01T00:00:00Z
  executor = "docker"
  [runners.cache]
    MaxUploadedArchiveSize = 0
  [runners.docker]
    tls_verify = true
    image = "alpine:3"
    privileged = true
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/certs/client", "/cache"]
    shm_size = 0

Used GitLab Runner version

gitlab-runner --version Version: 16.2.1 Git revision: 674e0e29 Git branch: 16-2-stable GO version: go1.20.5 Built: 2023-08-08T00:08:26+0000 OS/Arch: linux/amd64

Possible fixes

Adjust the schema to make ulimit optional.