Add warning for relative paths in source-filesystem.

Author: Fabian Schultz

packages/gatsby-source-filesystem/src/gatsby-node.js

@@ -1,5 +1,6 @@
 const chokidar = require(`chokidar`)
 const fs = require(`fs`)
+const path = require(`path`)
 const { Machine } = require(`xstate`)
 
 const { createFileNode } = require(`./create-file-node`)
@@ -86,6 +87,19 @@ See docs here - https://www.gatsbyjs.org/packages/gatsby-source-filesystem/
       `)
   }
 
+  // Validate that the path is absolute.
+  // Absolute paths are required to resolve images correctly.
+  if (!path.isAbsolute(pluginOptions.path)) {
+    reporter.warn(`The path passed to gatsby-source-filesystem is relative:
+
+${pluginOptions.path}
+
+It is recommended to use an absolute path.
+
+See docs here - https://www.gatsbyjs.org/packages/gatsby-source-filesystem/
+    `)
+  }
+
   const fsMachine = createFSMachine()
   let currentState = fsMachine.initialState
   let fileNodeQueue = new Map()