chore: add Docker env example and un-ignore it (#597)

Add .env.docker.example as a template for Docker-based local
development, and add a negation pattern to .gitignore so the
example file is tracked while real .env* files stay ignored.

Signed-off-by: Fabian Mossberg <fabian@hejfaktura.se>
Co-authored-by: Jakob Wennberg <jakob.wennberg@gmail.com>

.env.docker.example

@@ -0,0 +1,8 @@
+NEXT_PUBLIC_SUPABASE_URL=https://your-project.supabase.co
+NEXT_PUBLIC_SUPABASE_ANON_KEY=your-anon-key
+SUPABASE_SERVICE_ROLE_KEY=your-service-role-key
+NEXT_PUBLIC_APP_URL=https://your-domain.com
+CRON_SECRET=generate-a-random-secret
+
+# Self-hosted (Docker) flag: disables application-side MFA enforcement.
+NEXT_PUBLIC_SELF_HOSTED=true