How do I run PHPUnit tests?
I believe the problem is that you are using the sample phpunit.xml.dist instead of copying it to phpunit.xml and then making a few teaks for your installation.
Part of the phpunit.xml file is setting up the autoloader to find test classes and modules (they are not in the normal autoloader). And you tend to get some really weird errors if you don't set up the DSN for the database (to run Kernel and Functional tests).
I would do the following (adjust paths as needed ; I'm looking at a core-dev project right now and not a live site).

In your install, do composer require --dev drupal/core-dev:~9.1.0. You may want to tweak the version number, depending on you you have core defined.
Make sure you are running the binary from vendor/bin in your install.  I suggest looking into direnv.
Copy phpunit.xml.dist to phpunit.xml and edit

SIMPLETEST_BASE_URL
SIMPLETEST_DB
BROWSERTEST_OUTPUT_DIRECTORY
to reflect your installation.



Then cd to core, and run
../vendor/bin/phpunit -c phpunit.xml --testsuite unit

This should start to loop through the Unit tests. Abort that if they look like they are working, and then do
../vendor/bin/phpunit -c phpunit.xml --testsuite kernel

Then, if that looks OK you can start to run your own tests.
All of that said, it is much easier to run individual tests through PhpStorm. You just need to got through the preferences to point to the right PHP interpreter, and set a few things for phpunit.
In your case, it may be better to just start the drupal:9.1.0 container, and then shell into it for initial testing before updating the Dockerfile.