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/.env.example

@@ -0,0 +1,14 @@
+# Copy to .env (untracked) and adjust if the defaults in docker-compose.yml don't fit — none of
+# these need to be set for the default `mc-1_12_2` service to work as-is.
+
+# MC_1_12_2_PORT=25565
+# MC_1_12_2_MEMORY=3G
+# MC_1_12_2_ONLINE_MODE=FALSE
+
+# MC_1_7_10_PORT=25566
+# MC_1_7_10_MEMORY=2G
+# MC_1_7_10_ONLINE_MODE=FALSE
+
+# MC_NEOFORGE_26_1_PORT=25567
+# MC_NEOFORGE_26_1_MEMORY=3G
+# MC_NEOFORGE_26_1_ONLINE_MODE=FALSE