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

chart/readme.md

@@ -0,0 +1,44 @@
+# Introduction
+
+This is a [Helm](https://helm.sh/) chart for installing Mastodon into a
+Kubernetes cluster.  The basic usage is:
+
+```
+cp values.yaml.template values.yaml
+edit values.yaml # configure required settings
+helm dep update
+helm upgrade --install my-mastodon ./
+```
+
+This chart has been tested on Helm 3.0.1 and above.
+
+# Configuration
+
+The variables that _must_ be configured are:
+
+- `ingress.hostname`; even if you aren’t using an Ingress, this value is used to
+  set `LOCAL_DOMAIN`.
+
+- password and keys in the `secrets`, `postgresql`, and `redis` groups; if
+  left blank, some of those values will be autogenerated, but will not persist
+  across upgrades.
+
+- SMTP settings for your mailer in the `smtp` group.
+
+# Missing features
+
+Currently this chart does _not_ support:
+
+- Hidden services
+- S3/Minio/GCS
+- Single Sign-On
+- Swift
+- configurations using `WEB_DOMAIN`
+
+# Upgrading
+
+Because database migrations are managed as a Job separate from the Rails and
+Sidekiq deployments, it’s possible they will occur in the wrong order.  After
+upgrading Mastodon versions, it may sometimes be necessary to manually delete
+the Rails and Sidekiq pods so that they are recreated against the latest
+migration.