Move to new Airplane download system (#933)

Co-authored-by: Jawa-Juice <Jawa-Juice@users.noreply.github.com>

README.md

@@ -145,7 +145,7 @@ By default, the container will download the latest version of the "vanilla" [Min
       * [Enabling Autopause](#enabling-autopause)
    * [Running on RaspberryPi](#running-on-raspberrypi)
 
-<!-- Added by: runner, at: Tue Jun 22 23:39:21 UTC 2021 -->
+<!-- Added by: runner, at: Wed Jun 23 01:36:53 UTC 2021 -->
 
 <!--te-->
 
@@ -450,15 +450,18 @@ An [Airplane](https://github.com/TECHNOVE/Airplane) server, which is a fork of T
 
     -e TYPE=AIRPLANE
 
-> **NOTE** only `VERSION=LATEST` is supported
+> NOTE: The `VERSION` variable is used to select an Airplane branch to download from. The available options are "LATEST" "1.16" "1.17" and "PURPUR"
+
+> NOTE: The 1.17 branch is currently in beta. Use at your own risk!
 
 Extra variables:
+- `AIRPLANE_BUILD=lastSuccessfulBuild` : set a specific Airplane build to use
 - `FORCE_REDOWNLOAD=false` : set to true to force the located server jar to be re-downloaded
 - `USE_FLARE_FLAGS=false` : set to true to add appropriate flags for the [Flare](https://blog.airplane.gg/flare) profiler
 
 ### Running a Purpur server
 
-A [Purpur](https://purpur.pl3x.net/) server, which is "a drop-in replacement for Paper servers designed for configurability, new fun and exciting gameplay features, and high performance built on top of Tuinity.".
+A [Purpur](https://purpur.pl3x.net/) server, which is "a drop-in replacement for Paper servers designed for configurability, new fun and exciting gameplay features, and high performance built on top of Tuinity."
 
     -e TYPE=PURPUR