Composer cache gets cleared everytime

### Preconditions
1. Magento Cloud Docker with PHP >= 8.1

### Steps to reproduce
1. Executing any Docker command within a PHP container such as `bin/magento-docker bash`
2. Check out the output

### Expected result
1. No cache is cleared

### Actual result
1. Full composer cache gets cleared

### Additional information

This is due to the fact that the `docker-entrypoint.sh` has the following lines now:

```sh
# Configure composer
[ ! -z "${COMPOSER_VERSION}" ] && \
composer clearcache && \
composer self-update $COMPOSER_VERSION
```

because of that, Composer's cache is always cleared when the COMPOSER_VERSION environment variable is set.

This is very annoying since by default the local (host machine) Composer cache directory is a volume to the Docker's Composer cache directory, meaning that Magento Cloud Docker would clear the entire Composer cache everytime we run any command that runs the PHP container...

Start with docker-entrypoint.sh and inspect the Composer setup around COMPOSER_VERSION. Reproduce the issue with bin/magento-docker bash in a PHP container, then verify that invoking Docker commands no longer clears the shared Composer cache while the requested Composer version behavior still works.

Written by the indexing model from the issue text.