Update healthcheck to work with PORT variable

Dockerfile

@@ -54,9 +54,10 @@ COPY --link /public ./public
 COPY --link --from=builder /app/.next/standalone ./
 COPY --link --from=builder /app/.next/static/ ./.next/static/
 
+EXPOSE 3000
+ENV PORT='3000'
+
 HEALTHCHECK --interval=12s --timeout=12s --start-period=30s \  
     CMD node ./healthcheck.js
 
-EXPOSE 3000
-ENV PORT 3000
 CMD ["node", "server.js"]

healthcheck.js

@@ -1,7 +1,10 @@
 var http = require("http");
+
+const PORT = process.env.PORT || "3000";
+
 var options = {
   host: "localhost",
-  port: "3000",
+  port: PORT,
   timeout: 2000,
 };
 var request = http.request(options, (res) => {
@@ -16,4 +19,4 @@ request.on("error", function (err) {
   console.log("ERROR");
   process.exit(1);
 });
-request.end();
+request.end();