fix: exclude .env files from Docker image

The .dockerignore was missing .env files, which caused local development
environment variables (including test OIDC configuration) to be baked
into Docker images.

This resulted in Google login appearing in production Docker containers
even when OIDC wasn't configured via docker-compose environment
variables.

Now .env files are properly excluded from Docker builds.

.dockerignore

@@ -12,6 +12,8 @@
 *.swp
 *.swo
 *~
+.env
+**/.env
 **/.env.local
 **/.env.development
 **/.env.test