Improve Dockerfile layers and image building

[peter279k]

Changed log

• To reduce the Docker image layers, using the RUN command once instead.

Here is the reference about above issue: https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#minimize-the-number-of-layers

• When building the Docker image, it seems that the posix extension has been already loaded.
  I think the docker-php-ext-install posix command can be removed. The related warning message is as follows:

........
See any operating system documentation about shared libraries for
more information, such as the ld(1) and ld.so(8) manual pages.
----------------------------------------------------------------------

Build complete.
Don't forget to run 'make test'.

Installing shared extensions:     /usr/local/lib/php/extensions/no-debug-non-zts-20200930/

warning: posix (posix.so) is already loaded!
........