feat: add docker environment to run the checks locally

[YvesCesar]

Why

Running the checks locally depends on the host's PHP and OpenSSL, and on distributions where those differ from CI the suite fails for reasons that have nothing to do with the library. On Fedora 44 (PHP 8.5.9, OpenSSL 3.5.7) none of the three checks behave like they do on CI:

• composer run test:unit fails with 6 errors and 2 failures, all of them setCertificate(): Argument #1 must be of type string, null given. The test helper calls openssl_csr_sign() without digest_alg, so PHP signs the generated certificate with SHA-1, and the distribution's crypto policy rejects it (rh-allow-sha1-signatures = no, error:03000098 ... invalid digest). The certificate comes back null. This affects Fedora and RHEL 9+ by default.
• composer run psalm does not run at all. Psalm 6.5.0 turns PHP 8.5's SplObjectStorage::attach() deprecation into an uncaught Throwable and crashes before analysing anything. Silencing deprecations does not help, since the crash comes from Psalm's own error handler.
• example/index.php dies with a fatal error, because PharData exceeds the default 128M memory limit just to open the JRE tarball. The fatal skips the catch, so the temporary .pfx is left behind with its private key.

[vitormattos]

I approved the PR but send only a small suggestion that don't block the merge.

Fell free to choose what you prefer to do.