More Docker changes:

* disable in-network updates
* persistent directory for snark downloads
* listen on port 12345 for i2np connections
* restore multi-stage build
* use better ant target
* allow passing -Xmx via the JVM_XMX variable

Many thanks to LoveIsGrief for the suggestions!

docker/rootfs/i2p/i2psnark.config

@@ -0,0 +1,12 @@
+# NOTE: This I2P config file must use UTF-8 encoding
+#
+# If you have a 'split' directory installation, with configuration
+# files in ~/.i2p (Linux), %LOCALAPPDATA%\I2P (Windows),
+# or /Users/(user)/Library/Application Support/i2p (Mac), be sure to
+# edit the file in the configuration directory, NOT the install directory.
+# When running as a Linux daemon, the configuration directory is /var/lib/i2p
+# and the install directory is /usr/share/i2p .
+# When running as a Windows service, the configuration directory is \ProgramData\i2p
+# and the install directory is \Program Files\i2p .
+#
+i2psnark.dir=/i2psnark

docker/rootfs/i2p/router.config

@@ -0,0 +1,4 @@
+router.updateDisabled=true
+i2np.ntcp.port=12345
+i2np.udp.port=12345
+i2np.udp.internalPort=12345

docker/rootfs/startapp.sh

@@ -1,6 +1,13 @@
 #!/bin/sh
 set -e
 
+if [ -z $JVM_XMX ]; then
+    echo "*** Defaulting to 512MB JVM heap limit"
+    echo "*** You can override that value with the JVM_XMX variable"
+    echo "*** (for example JVM_XMX=256m)"
+    JVM_XMX=512m
+fi
+
 # Explicitly define HOME otherwise it might not have been set
 export HOME=/i2p
 
@@ -15,7 +22,7 @@ for jar in `ls lib/*.jar`; do
     CLASSPATH=${CLASSPATH}:${jar}
 done
 
-JAVAOPTS="-Djava.net.preferIPv4Stack=false -Djava.library.path=${I2P}:${I2P}/lib -Di2p.dir.base=${I2P} -Di2p.dir.config=${HOME}/.i2p -DloggerFilenameOverride=logs/log-router-@.txt"
+JAVAOPTS="-Djava.net.preferIPv4Stack=false -Djava.library.path=${I2P}:${I2P}/lib -Di2p.dir.base=${I2P} -Di2p.dir.config=${HOME}/.i2p -DloggerFilenameOverride=logs/log-router-@.txt -Xmx$JVM_XMX"
 
 java -cp "${CLASSPATH}" ${JAVA_OPTS} net.i2p.router.RouterLaunch