Jenkins dokku workflow

1. `Dockerfile` with content:
```
  FROM jenkinsci/jenkins:2.9
```
1. Create `jenkins` app:
```
  $ dokku apps:create jenkins
  -----> Application name is jenkins
  Creating jenkins... done
```
1. Map port 80 to Jenkins default port (8080):
```
  $ dokku proxy:ports-add jenkins http:80:8080
  -----> Setting config vars
          DOKKU_PROXY_PORT_MAP: http:80:8080
  -----> Setting config vars
          DOKKU_NGINX_PORT: 80
  -----> Configuring dev.kozminski.me...(using built-in template)
  -----> App jenkins has not been deployed. Skipping nginx config creation
```
1. Add external storage to jenkins app:
```
  $ dokku storage:mount jenkins /var/lib/dokku/data/storage/jenkins:/var/jenkins_home
```
1. Add remote:
```
  $ git remote add dokku dokku@[HOST]:jenkins
```
1. Deploy:
```
  $ git add .
```
```
  $ git commit -m 'Jenkins'
  [master (root-commit) 3798ef9] Jenkins
   1 file changed, 1 insertion(+)
   create mode 100644 Dockerfile
```
```
  $ git push dokku master
  Counting objects: 3, done.
  Writing objects: 100% (3/3), 239 bytes | 0 bytes/s, done.
  Total 3 (delta 0), reused 0 (delta 0)
  -----> Cleaning up...
  -----> Building jenkins from dockerfile...
  remote: build context to Docker daemon 2.048 kB
  Step 1 : FROM jenkinsci/jenkins:2.9
   ---> a72096e8eaf5
  Successfully built a72096e8eaf5
  -----> Setting config vars
         DOKKU_DOCKERFILE_PORTS: 50000/tcp 8080/tcp
  -----> Releasing jenkins (dokku/jenkins:latest)...
  -----> Deploying jenkins (dokku/jenkins:latest)...
  -----> Attempting to run scripts.dokku.predeploy from app.json (if defined)
  -----> No Procfile found in app image
  -----> DOKKU_SCALE file not found in app image. Generating one based on Procfile...
  -----> New DOKKU_SCALE file generated
  =====> web=1
  -----> Attempting pre-flight checks
         For more efficient zero downtime deployments, create a file CHECKS.
         See http://dokku.viewdocs.io/dokku/checks-examples.md for examples
         CHECKS file not found in container: Running simple container check...
  -----> Waiting for 10 seconds ...
  -----> Default container check successful!
  -----> Running post-deploy
  -----> Attempting to run scripts.dokku.postdeploy from app.json (if defined)
  =====> renaming container (36082cc9c972) boring_fermat to jenkins.web.1
  -----> Creating new /home/dokku/jenkins/VHOST...
  -----> Configuring jenkins.dev.kozminski.me...(using built-in template)
  -----> Creating http nginx.conf
  -----> Running nginx-pre-reload
         Reloading nginx
  -----> Setting config vars
         DOKKU_APP_RESTORE: 1
  =====> Application deployed:
         http://jenkins.[HOST]

  To dokku@[HOST]:jenkins
   * [new branch]      master -> master
```