Add Symfony runtime injection documentation

Document how Symfony applications only need secrets injected during cache:warmup,
after which all commands use the compiled container from var/cache/prod/

Author: jszobody

README.md

@@ -39,10 +39,10 @@ composer require stechstudio/keep
 # Create template from existing secrets
 ./vendor/bin/keep template:add .env.template --stage=production
 
-# Use template with placeholders
-./vendor/bin/keep export --stage=production --template=.env.template --file=.env
+# Use template with placeholders to generate .env file
+./vendor/bin/keep export --stage=production --template=env/production.env --file=.env
 
-# Runtime injection - execute with secrets, no files created
+# Runtime injection - execute with secrets, no .env file created
 ./vendor/bin/keep run --vault=ssm --stage=production -- npm start
 ```
 

docs/guide/deployment/runtime-injection.md

@@ -39,6 +39,21 @@ php artisan queue:restart
 
 Once cached, Laravel completely ignores environment variables, reading everything from `bootstrap/cache/config.php`.
 
+## Symfony Applications
+
+Symfony compiles environment variables into the container during cache warmup. After compilation, all commands use the cached container:
+
+```bash
+# Inject secrets during container compilation (only injection needed!)
+keep run --vault=ssm --stage=production -- php bin/console cache:warmup --env=prod
+
+# All subsequent commands use compiled container - no injection required
+php bin/console doctrine:migrations:migrate --no-interaction
+php bin/console messenger:consume async
+```
+
+Once compiled, Symfony reads configuration from `var/cache/prod/` and never accesses environment variables.
+
 ## Node.js Applications
 
 ```bash