Use the deterministically built wasm

Has the Dockerfile output to a volume. Has the node use the wasm from the
volume, if it exists.

substrate/signals/pallet/src/lib.rs

@@ -142,6 +142,7 @@ pub mod pallet {
   }
 
   // 80% threshold
+  // TODO: Use 34% for halting a set (not 80%)
   const REQUIREMENT_NUMERATOR: u64 = 4;
   const REQUIREMENT_DIVISOR: u64 = 5;