Expose port count as _COUNT env varible

src/docker.js

@@ -293,6 +293,7 @@ async function createSubcontainer(app, name, cmd, options) {
         const hostIps = hostPort === 53 ? getAddressesForPort53() : [ '0.0.0.0', '::0' ]; // port 53 is special because it is possibly taken by systemd-resolved
 
         portEnv.push(`${portName}=${hostPort}`);
+        if (portCount > 1) portEnv.push(`${portName}_COUNT=${portCount}`);
 
         // docker portBindings requires ports to be exposed
         for (let i = 0; i < portCount; ++i) {