fix(country-map): re-include country-maps GeoJSON in Docker build context

.dockerignore excludes superset/static/assets/ wholesale because the
rest of that directory is webpack-generated at image build time. Our
country-map plugin's GeoJSON outputs are committed (not webpack-
generated), so they were getting silently dropped from every Docker
build — ephemeral PR previews 404'd on every map file fetch.

Add a `!` re-include for the country-maps subdirectory so the assets
ride along while leaving the rest of the webpack-managed exclusion
intact.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: Superset Dev

.dockerignore

@@ -43,6 +43,11 @@ superset-frontend/cypress-base/
 superset-frontend/coverage/
 superset-frontend/.temp_cache/
 superset/static/assets/
+# …but ship the committed Country Map GeoJSON outputs, which live under
+# this otherwise webpack-managed path. Without this re-include the
+# country-map plugin gets a 404 fetching every map file in dockerized
+# deployments (including ephemeral PR previews).
+!superset/static/assets/country-maps/
 superset-websocket/dist/
 venv
 .venv