Implement basic Velocity proxy

- Split entrypoint into functions for Minecraft and Velocity
- Implement Velocity Dockerfile based on JRE image
- Add velocity default build to ./builds
- Change error to warning for missing plugins.json build file

builds/velocity/.env

@@ -0,0 +1,18 @@
+# No EULA requirement to run Velocity
+VERSION=3.3.0-SNAPSHOT
+VELOCITY_TAG=${VERSION}
+
+# Run
+RUN_IMAGE=${VELOCITY_IMAGE:-localhost/velocity}
+RUN_TAG=${VERSION}
+
+########################
+# Extra image settings #
+########################
+#
+# JAVA_VERSION=latest
+# JRE_IMAGE=localhost/minecraft-jre
+# JRE_TAG=latest
+#
+# VELOCITY_IMAGE=localhost/velocity
+#