docker: update docker-compose file.

* Listen on port 3000 instead of port 80. Port 80 is prone to conflicts
  with other webservers.

* Use the production version of the Danbooru Docker image. It's less
  likely to have bugs than master.

* Fix the autoinstall script to work with `curl ... | sh`.

* Lower PUMA_WORKERS to 1 to reduce memory usage.

bin/danbooru

@@ -1,7 +1,7 @@
-#!/usr/bin/env bash
+#!/bin/sh
 #
 # This script starts Danbooru by installing Docker and Docker Compose, then
-# starting Danbooru in a container. Danbooru will be available at http://localhost.
+# starting Danbooru in a container. Danbooru will be available at http://localhost:3000.
 #
 # Usage:
 #
@@ -15,11 +15,11 @@
 #
 # This script is just a wrapper for that command.
 
-set -euxo pipefail
+set -eux
 
 # Check if program exists.
 has() {
-  type -p "$1" > /dev/null
+  type "$1" > /dev/null 2>&1
 }
 
 # Install Docker and Docker Compose if they're not installed already.