Add Buildable to the compose file

This is the only bot that will have write access to the rakudo git repo,
moarvm git repo and the builds directory.

I'm giving it a bit more memory just in case. I'm actually not sure how
much peak memory it really needs.

compose.yaml

@@ -1,6 +1,22 @@
 version: "3.7"
 
 services:
+  buildable:
+    image: postmodern.alexdaniel.org/whateverable:latest
+    command: sake debug:buildable
+    volumes:
+      - /home/whateverable/data/rakudo-moar:/srv/data/rakudo-moar
+      - /home/whateverable/data/moarvm:/srv/data/moarvm
+      - /home/whateverable/data/builds:/srv/data/builds
+    secrets:
+      - config
+    networks:
+      - caddy
+    deploy:
+      replicas: 1
+      resources:
+        limits:
+          memory: 4.0G
 
   bisectable:
     image: postmodern.alexdaniel.org/whateverable:latest