Add release selection for the GitHub-released server types (#4339)

tests/setuponlytests/poseidon-release/docker-compose.yml

@@ -0,0 +1,12 @@
+services:
+  mc:
+    restart: "no"
+    image: ${IMAGE_TO_TEST:-itzg/minecraft-server}
+    environment:
+      EULA: "TRUE"
+      SETUP_ONLY: "TRUE"
+      TYPE: POSEIDON
+      VERSION: "b1.7.3"
+      POSEIDON_RELEASE: "1.1.12-260720-1033-5303bd5"
+    volumes:
+      - ./data:/data

tests/setuponlytests/poseidon-release/verify.sh

@@ -0,0 +1 @@
+mc-image-helper assert fileExists "/data/poseidon-craftbukkit-*.jar"