Add and document docker-compose file

README.md

-# New Project
+# Helpful contact form
 
 ## How to start
 
+### Using VSCode
+
 You can use any editor you like, but the following instructions lead to a 
 containerised dev environment using VSCode or Codium:
 
@@ -20,6 +22,11 @@ Next: `Ctrl+P`, start typing: `Reopen in Container` and hit `Return`.
 
 You can now use any of the scripts defined in `package.json`, described below.
 
+### Using any other editor
+
+You can run the development container manually like this: `docker-compose up`.
+This will run `npm start` internally, exposed to port `8080` on localhost.
+
 ## Available Scripts
 
 ### npm start

docker-compose.yml

+version: "3.7"
+
+services:
+  nodejs:
+    build:
+      context: .
+      dockerfile: .devcontainer/Dockerfile
+    command: >
+      sh -c "pnpm install &&
+             npm start"
+    volumes:
+      - .:/workspaces/outage-form
+    user: "1000:1000"
+    ports:
+      - "8080:8080"