Release 0.18.0

# History

## 0.18.0

Install/update via the bootstrap script:

```shell
wget https://raw.githubusercontent.com/dokku/dokku/v0.18.0/bootstrap.sh
sudo DOKKU_TAG=v0.18.0 bash bootstrap.sh
```

### Bug Fixes

- #3627: @josegonzalez Make image removal synchronous
- #3618: @josegonzalez Ensure the dokku-retire timer is properly installed
- #3614: @alexquick Validate args for config:set and config:unset
- #3605: @josegonzalez Handle case where there are empty newlines in the authorized_keys file
- #3603: @josegonzalez Drop extra % sign in common.LogVerboseQuiet
- #3597: @josegonzalez Allow default trace function to work

### New Features

- #3628: @josegonzalez Handle file copying in a secure and reliable fashion
- #3630: @josegonzalez Fix issue where push warning on bad branch was skipped
- #3629: @josegonzalez Avoid calling the user-auth trigger where possible
- #3626: @josegonzalez Builder plugins
- #3599: @josegonzalez Scope docker-cleanup to specific app
- #3589: @michaelshobbs Allow running Dokku in Docker
- #3607: @josegonzalez Purge cache using herokuish image
- #3602: @alexymik Create a 502 error page to automatically refresh if backend status changes
- #3600: @josegonzalez Refactor IsImageHerokuishBased to match shell version

### Documentation

- #3625: @josegonzalez Remove old reference to SPONSORS.md
- #3619: @josegonzalez Cleanup plugin creation docs
- #3612: @jayjun Improve testing docs
- #3613: @Lyelt Remove all uses of proxy_set_header Connection "upgrade"
- #3596: @josegonzalez Add missing hooks to events plugin and plugin triggers docs

### Tests

- #3610: @jayjun Correct Bats path in single tests

docs/assets/style.css

@@ -39,13 +39,13 @@ h1 {
   font-size: 1.6em;
 }
 .header .navbar-brand a {
-  background-image: url(https://cdn.jsdelivr.net/gh/dokku/dokku@v0.17.9/docs/assets/dokku.png);
+  background-image: url(https://cdn.jsdelivr.net/gh/dokku/dokku@v0.18.0/docs/assets/dokku.png);
   text-indent: 40px;
 }
 .blurb {
   color: #424242;
   background-color: #ededed;
-  background-image: url(https://cdn.jsdelivr.net/gh/dokku/dokku@v0.17.9/docs/assets/gplaypattern.png);
+  background-image: url(https://cdn.jsdelivr.net/gh/dokku/dokku@v0.18.0/docs/assets/gplaypattern.png);
   padding: 45px 0;
   text-align: center;
 }