Add Helm chart (#14090)

* add Helm chart

known issues/future work:

- SSO is unsupported

- S3/Minio/GCS is unsupported

- Swift is unsupported

- WEB_DOMAIN is unsupported

- Tor is unsupported

* helm: clarify how LOCAL_DOMAIN is set

* helm: add chart description

* helm: make DB_POOL and Sidekiq concurrency configurable

* helm: only enforce pod affinity when using ReadWriteOnce

* helm: clarify compatibility

* helm: clean up application variables

* helm: add job to create initial admin

.gitignore

@@ -43,6 +43,11 @@ postgres
 redis
 elasticsearch
 
+# ignore Helm lockfile, dependency charts, and local values file
+chart/Chart.lock
+chart/charts/*.tgz
+chart/values.yaml
+
 # Ignore Apple files
 .DS_Store