feat(gatsby-plugin-sass): add option to enable resolve-url-loader (#14272)

* Using resolve-url-loader if configured in the options

* Documenting how to use resolve-url-plugin plugin

* Fixing small typo

* Fixing small typo

* Fixing prettier checks

* Using backtick quotes for strings

* resolve-url-loader needs sass-loader to have sourceMap activated

* Updating documentation

* Updating documentation about sourceMap

* chore: format

* Update README.md

* Update README.md

Running prettier on README

* chore: format

Author: carlosruana

packages/gatsby-plugin-sass/README.md

@@ -116,9 +116,62 @@ in the plugin options.
 
 This plugin resolves `url()` paths relative to the entry SCSS/Sass file not – as might be expected – the location relative to the declaration. Under the hood, it makes use of [sass-loader](https://github.com/webpack-contrib/sass-loader/blob/master/README.md#problems-with-url) and this is documented in the [readme](https://github.com/webpack-contrib/sass-loader/blob/master/README.md#problems-with-url).
 
-Using [resolve-url-loader](https://github.com/bholloway/resolve-url-loader) may provide a workaround, but at present this is not in the build and implementation would demand customization.
+Using [resolve-url-loader](https://github.com/bholloway/resolve-url-loader) provides a workaround, if you want to use relative url just install the plugin and then add it to your sass plugin options configuration.
 
-<!-- TODO link to a plugin that adds resolve-url-loader -->
+First:
+
+```javascript
+  npm install resolve-url-loader --save-dev
+  or
+  yarn add resolve-url-loader --dev
+```
+
+And then:
+
+```javascript
+plugins: [
+  {
+    resolve: "gatsby-plugin-sass",
+    options: {
+      useResolveUrlLoader: true,
+    },
+  },
+]
+```
+
+You can also configure resolve-url-plugin providing some options (see plugin documentation for all options https://github.com/bholloway/resolve-url-loader):
+
+```javascript
+plugins: [
+  {
+    resolve: "gatsby-plugin-sass",
+    options: {
+      useResolveUrlLoader: {
+        options: {
+          debug: true,
+        },
+      },
+    },
+  },
+]
+```
+
+NOTE that adding resolve-url-loader will use `sourceMap: true` on sass-loader (as it is required for the plugin to work), you can then activate/deactivate source-map for sass files in the plugin:
+
+```javascript
+plugins: [
+  {
+    resolve: "gatsby-plugin-sass",
+    options: {
+      useResolveUrlLoader: {
+        options: {
+          sourceMap: true, //default is false
+        },
+      },
+    },
+  },
+]
+```
 
 ## Breaking changes history
 

packages/gatsby-plugin-sass/src/gatsby-node.js

@@ -2,7 +2,7 @@ import resolve from "./resolve"
 
 exports.onCreateWebpackConfig = (
   { actions, stage, rules, plugins, loaders },
-  { cssLoaderOptions = {}, postCssPlugins, ...sassOptions }
+  { cssLoaderOptions = {}, postCssPlugins, useResolveUrlLoader, ...sassOptions }
 ) => {
   const { setWebpackConfig } = actions
   const PRODUCTION = stage !== `develop`
@@ -11,7 +11,7 @@ exports.onCreateWebpackConfig = (
   const sassLoader = {
     loader: resolve(`sass-loader`),
     options: {
-      sourceMap: !PRODUCTION,
+      sourceMap: useResolveUrlLoader ? true : !PRODUCTION,
       ...sassOptions,
     },
   }
@@ -27,6 +27,12 @@ exports.onCreateWebpackConfig = (
           sassLoader,
         ],
   }
+  if (useResolveUrlLoader && !isSSR) {
+    sassRule.use.splice(-1, 0, {
+      loader: `resolve-url-loader`,
+      options: useResolveUrlLoader.options ? useResolveUrlLoader.options : {},
+    })
+  }
   const sassRuleModules = {
     test: /\.module\.s(a|c)ss$/,
     use: [