read GATSBY_* env vars and feed to webpack (gatsbyjs#2057)

* read GATSBY_* env vars and feed to webpack

* explanation of GATSBY* env vars

* typo

* added syntax highlighting to doc

docs/docs/environment-variables.md

@@ -21,6 +21,27 @@ API_URL=https://example.com/api
 
 These variables will be available to your site as `process.env.API_URL`.
 
+In addition any variable in the environment prefixed with `GATSBY_` will be available.
+
+## Example
+
+```
+GATSBY_ASSETS_URL=http://s3.amazonaws.com/bucketname
+```
+
+```jsx
+// usage
+render() {
+  return (
+    <div>
+      <img src={`${process.env.GATSBY_ASSETS_URL}/logo.png`} />
+    </div>
+  )
+}
+```
+
+
+
 > You can not override certain environment variables as some are used internally for optimizations during build
 
 Reserved environment variables:

packages/gatsby/src/utils/webpack.config.js

@@ -64,11 +64,18 @@ module.exports = async (
       return acc
     }, {})
 
+    const gatsbyVarObject = Object.keys(process.env).reduce((acc, key) => {
+      if (key.match(/^GATSBY_/)) {
+        acc[key] = JSON.stringify(process.env[key])
+      }
+      return acc
+    }, {})
+
     // Don't allow overwriting of NODE_ENV, PUBLIC_DIR as to not break gatsby things
     envObject.NODE_ENV = JSON.stringify(env)
     envObject.PUBLIC_DIR = JSON.stringify(`${process.cwd()}/public`)
 
-    return envObject
+    return Object.assign(envObject, gatsbyVarObject)
   }
 
   debug(`Loading webpack config for stage "${stage}"`)