Dynamically download modpacks at runtime if so desired

Dockerfile

@@ -17,6 +17,7 @@ RUN apk add curl openjdk8-jre &&\
 
 # The second stage is the actual container
 FROM openjdk:8
+RUN apt-get update && apt-get install rsync -y
 WORKDIR /minecraft
 COPY --from=build /minecraft .
 CMD [ "bash", "start-server.sh" ]

README.md

@@ -28,6 +28,7 @@ variable|description
 ---|---
 `JRE_XMX`|Maximum amount of heap passed to the main Minecraft process
 `JRE_XMS`|Minimum heap size passed to the main Minecraft process
+`FORGE_PACK_ZIP`|If provided, the URL to a zip or tar.gz file that contains the modpack that needs to be installed. Will be intelligently extracted into the server directory through the magic of `find`.
 `ARGS`|Any additional arguments to be passed to the JVM
 
 ## Useful Arguments
@@ -47,6 +48,14 @@ enable-command-block=true
 spawn-protection=0
 ```
 
+## FAQ
+
+**Why do you download the pack at runtime?**
+
+It allows for transparent upgrades without having to build a whole new container. You may lose container consistency, but you gain simplicity in definition.
+
+Of course, nothing prevents you from just not using this magic variable and instead `FROM`ing this container and building your own pack.
+
 ## License
 
 MIT

start-server.sh

@@ -5,6 +5,20 @@
 #
 # Distributed under terms of the MIT license.
 #
+set -e
+
+# Download and extract a pack zip, if one is provided
+if [ -n "$FORGE_PACK_ZIP" ]; then
+	echo "Downloading pack: $FORGE_PACK_ZIP"
+	tmpdir="$(mktemp -d)"
+	pushd "$tmpdir" > /dev/null 2>&1
+	curl -L "$FORGE_PACK_ZIP" -o pack.zip
+	unzip pack.zip
+	directory="$(find . -type d -iname "mods" -execdir pwd \; | sort -n | head -n 1)"
+	echo "Found modpack directory: $directory"
+	rsync -av "$directory"/ /minecraft/
+	popd > /dev/null 2>&1
+fi
 
 # Entrypoint will be server.jar
 args="-jar server.jar nogui"