add build sources to hcl examples

website/pages/docs/builders/docker.mdx

@@ -92,6 +92,10 @@ source "docker" "example" {
     image = "ubuntu"
     commit = true
 }
+
+build {
+  sources = ["source.docker.example"]
+}
 ```
 
 </Tab>
@@ -429,6 +433,10 @@ source "docker" "windows" {
     windows_container = true
     commit = true
 }
+
+build {
+  sources = ["source.docker.example"]
+}
 ```
 
 </Tab>