Fixes pure Dockerfile deployment failing with 'env file not found' error.
The deploy_simple_dockerfile() method was missing the call to
save_runtime_environment_variables() which creates the .env file
needed during the rolling update phase. This call is present in
all other deployment methods (dockerfile, dockercompose, nixpacks,
static) but was missing here.
This ensures the .env file exists when docker compose tries to
use --env-file during the rolling update.