[mc] Add PROXY env var option

For #177

minecraft-server/README.md

@@ -732,3 +732,11 @@ In order to adapt to differences in `UID` and `GID` settings the entry script wi
 General JVM options can be passed to the Minecraft Server invocation by passing a `JVM_OPTS`
 environment variable. Options like `-X` that need to proceed general JVM options can be passed
 via a `JVM_XX_OPTS` environment variable.
+
+### HTTP Proxy
+
+You may configure the use of an HTTP/HTTPS proxy by passing the proxy's URL via the `PROXY`
+environment variable. In [the example compose file](docker-compose-proxied.yml) it references 
+a companion squid proxy by setting the equivalent of
+
+    -e PROXY=proxy:3128

minecraft-server/docker-compose-proxied.yml

@@ -0,0 +1,15 @@
+version: '3'
+
+services:
+  mc:
+    build: .
+    ports:
+      - 25565:25565
+    environment:
+      EULA: "TRUE"
+#      TYPE: SPIGOT
+#      VERSION: 1.11.2
+      PROXY: proxy:3128
+  proxy:
+    image: sameersbn/squid
+

minecraft-server/start-configuration

@@ -29,6 +29,13 @@ if ! touch /data/.verify_access; then
   exit 2
 fi
 
+if [[ $PROXY ]]; then
+    export http_proxy="$PROXY"
+    export https_proxy="$PROXY"
+    echo "INFO: Giving proxy time to startup..."
+    sleep 5
+fi
+
 export SERVER_PROPERTIES=/data/server.properties
 export FTB_DIR=/data/FeedTheBeast
 export VERSIONS_JSON=https://launchermeta.mojang.com/mc/game/version_manifest.json