Add itzg-based docker-compose for live-testing the mod against real MC servers

Three services (mc-1_12_2 default, mc-1_7_10/mc-neoforge-26_1 opt-in via
Compose profiles) using itzg/docker-minecraft-server, with mods/config bind-
mounted per loader/version (mods/<loader>/<version>/) so jars can be dropped
in and swapped without touching the compose file. Verified against real
containers: fixed a WSL docker credential-helper misconfig blocking pulls,
and pinned legacy Forge services to the java8 image tag after itzg's
:latest (Java 25) crashed LaunchWrapper with a ClassCastException — mc-1_12_2
now boots cleanly to "Done" on java8.

test/.gitignore

@@ -0,0 +1,7 @@
+# Real jars/configs/world data dropped in here for local live-testing — never committed. The
+# per-loader-per-version directories themselves are tracked via .gitkeep so `docker compose up`
+# has somewhere to bind-mount without a manual `mkdir` first.
+mods/*/*/*
+config/*/*/*
+!mods/*/*/.gitkeep
+!config/*/*/.gitkeep