Fix registry auth to send base64url-encoded X-Registry-Auth header

[clue]

This changeset fixes the registry authentication for imageCreate() and imagePush() to send the X-Registry-Auth header base64url-encoded as Docker expects. It previously used standard base64, so whenever the encoded credentials happened to contain a + or /, Docker could not decode the header and ignored the credentials. This only affected some credentials, which likely explains why it went unnoticed since the very first release in 2014.

Refs https://github.com/moby/moby/blob/docker-v29.9.1/api/pkg/authconfig/authconfig.go

[Copilot]

🟢 Approval recommended

The focused encoding fix matches Docker’s requirements, and the regression test covers both affected characters.

No open issues 🎉

What changed in this PR

Fixes registry authentication for imageCreate() and imagePush() by sending Docker-compatible base64url headers.

Changes:

• Updates the shared encoder and its documentation.
• Adds a regression test covering both character substitutions.

File	Description
tests/​ClientTest.php	Verifies the encoded registry-auth header during image push.
src/​Client.php	Converts base64 + and / to base64url - and _.

_{Review effort: Balanced}