Since there are still no official images for PHP 7.4 (RC4) based on Debian stretch I have created some. The Swoole image also has several other modules. There are also some bash scripts for container start in the Github repo.
The changes in the Dockerfile compared to 7.3 include the addition of −−with-pear (Pear is being deprecated in PHP 7.4 and seems it is going to be removed in PHP 8) and the addition of the oniguruma library (libonig-dev).
22/10/2019 Update: The images now support SSL and Swoole can be run with HTTP2 support. Also swoole_postgresql is also compiled in.