Containerise socket server

See https://github.com/excalidraw/excalidraw/issues/1772#issuecomment-652043326

Dockerfile

+FROM node:14-alpine
+
+WORKDIR /excalidraw-room
+
+COPY package.json yarn.lock ./
+COPY scripts ./scripts
+RUN SKIP_YARN_POSTINSTALL=true yarn
+
+COPY tsconfig.json ./
+COPY src ./src
+RUN yarn build
+
+EXPOSE 80
+CMD ["yarn", "start"]

package.json

     "build": "tsc",
     "fix": "yarn prettier --write",
     "lint": "yarn prettier --list-different",
-    "postinstall": "npm run build",
+    "postinstall": "node scripts/postinstall.js",
     "prettier": "prettier \"**/*.{ts,md,json,yaml,yml}\"",
     "start": "node dist/index.js",
     "test": "yarn lint"
     "typescript": "3.9.5"
   },
   "eslintConfig": {
+    "env": {
+      "es6": true
+    },
     "extends": [
       "prettier"
     ],

scripts/postinstall.js

+// There doesn't appear to be a way to skip `postinstall`,
+// as doing `yarn --ignore-scripts` could prevent native packages from building:
+// https://github.com/yarnpkg/yarn/issues/4100
+
+const { execSync } = require("child_process");
+
+if (process.env.SKIP_YARN_POSTINSTALL !== "true") {
+  execSync("yarnpkg build", { stdio: "inherit" });
+}

tsconfig.json

     "resolveJsonModule": true,
     "isolatedModules": true,
     "outDir": "dist"
-  }
+  },
+  "include": ["src/**/*"]
 }