feat(config): Add env option for base url (#60)

Author: moop-moop

README.md

@@ -102,3 +102,11 @@ package = "@netlify/plugin-sitemap"
   changeFreq = "daily"
   priority = 0.5
 ```
+
+### Set base URL from environment variable rather than plugin input
+
+You can include an environment variable (`NETLIFY_PLUGIN_SITEMAP_BASEURL`) in your Netlify site to set the base URL that will be used by the plugin. This option is useful if the `baseUrl` plugin input can't be used.
+Example use case: different Netlify sites built from the same repository and don't/can't have custom domains.
+
+Priority of base URL assignment:
+plugin input `baseUrl` -> env `NETLIFY_PLUGIN_SITEMAP_BASEURL` -> Netlify site default URL

index.js

@@ -19,7 +19,7 @@ const getBuildDir = ({ inputs, constants }) => {
 
 module.exports = {
   onPostBuild: async ({ constants, inputs, utils }) => {
-    const baseUrl = inputs.baseUrl || env.URL
+    const baseUrl = inputs.baseUrl || env.NETLIFY_PLUGIN_SITEMAP_BASEURL || env.URL
     const buildDir = getBuildDir({ inputs, constants })
 
     console.log('Creating sitemap from files...')