docs: document building the image locally for testing

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -45,6 +45,25 @@ cannot push to the package registry on this instance.
 - **Owner:** TODO — fill in once the token is generated (Settings → Applications → Generate New
   Token on the account that should own the published packages).
 
+## Building locally for testing
+
+The `Dockerfile`'s `GODOT_VERSION` and `BLENDER_URL` build args both have defaults, so a plain
+build with no args works for local testing:
+
+```sh
+docker build -t godot-ci-local .
+```
+
+Override either to test a specific combination, matching what `publish.yml` would resolve for a
+given night:
+
+```sh
+docker build \
+  --build-arg GODOT_VERSION=4.6.3 \
+  --build-arg BLENDER_URL=https://download.blender.org/release/Blender4.2/blender-4.2.3-linux-x64.tar.xz \
+  -t godot-ci-local .
+```
+
 ## Consuming this image
 
 In `Project4x`'s `.gitea/workflows/export-template.yml`, pin to a specific version (recommended,