Fix tests in Docker for 3.x

[denis-trofimov]

• The php:7.2-cli image is based on EOL Debian Buster; apt-get update fails against the default mirrors. Point apt at archive.debian.org and document the requirement in the README.

• if Docker runs an older PHP (for example 7.2 while dependencies were resolved on 7.4), PHPUnit can fail with hundreds of ParseErrors in vendor/ (often in packages used by Prophecy, such as phpdocumentor/reflection-docblock).

Composer (version 2.9.8) successfully installed to: /usr/local/bin/composer
Use it: php /usr/local/bin/composer

Installing dependencies from lock file (including require-dev)
Verifying lock file contents can be installed on current platform.
Your lock file does not contain a compatible set of packages. Please run composer update.

  Problem 1
    - phpdocumentor/reflection-docblock is locked to version 5.6.7 and an update of this package was not requested.
    - phpdocumentor/reflection-docblock 5.6.7 requires php ^7.4 || ^8.0 -> your php version (7.2.34) does not satisfy that requirement.
  Problem 2
    - phpdocumentor/type-resolver is locked to version 1.12.0 and an update of this package was not requested.
    - phpdocumentor/type-resolver 1.12.0 requires php ^7.3 || ^8.0 -> your php version (7.2.34) does not satisfy that requirement.
  Problem 3
    - phpstan/phpdoc-parser is locked to version 2.3.2 and an update of this package was not requested.
    - phpstan/phpdoc-parser 2.3.2 requires php ^7.4 || ^8.0 -> your php version (7.2.34) does not satisfy that requirement.
  Problem 4
    - phpspec/prophecy is locked to version v1.10.3 and an update of this package was not requested.
    - phpdocumentor/reflection-docblock 5.6.7 requires php ^7.4 || ^8.0 -> your php version (7.2.34) does not satisfy that requirement.
    - phpspec/prophecy v1.10.3 requires phpdocumentor/reflection-docblock ^2.0|^3.0.2|^4.0|^5.0 -> satisfiable by phpdocumentor/reflection-docblock[5.6.7].