Add missing build script to package.json and ignore root build directory

.gitignore

@@ -9,6 +9,7 @@ README.html
 addon.env
 
 src/web-ext-artifacts/*
+web-ext-artifacts
 
 # JetBrains IDE files
 .idea

package.json

@@ -36,6 +36,7 @@
     "url": "git+https://github.com/mozilla/multi-account-containers.git"
   },
   "scripts": {
+    "build": "web-ext build -s src/",
     "dev": "npm run remove-locales-github && web-ext run -s src/",
     "lint": "npm-run-all lint:*",
     "lint:addon": "./bin/addons-linter.sh",