👷(docker) add arm64 support and upgrade CI workflow steps (#11)

* 👷(docker) add arm64 platform support for image builds

Add QEMU and Buildx setup steps to enable multi-architecture Docker
image builds for both linux/amd64 and linux/arm64 platforms.

Signed-off-by: Stephan Meijer <me@stephanmeijer.com>

* ⬆️(ci) upgrade GitHub Actions workflow steps to latest versions

Update actions/setup-node from v4 to v6 and actions/cache from v4 to v5
for improved performance, security patches, and Node.js runtime compatibility.

Signed-off-by: Stephan Meijer <me@stephanmeijer.com>

---------

Signed-off-by: Stephan Meijer <me@stephanmeijer.com>

.github/workflows/calendars.yml

@@ -131,7 +131,7 @@ jobs:
           sudo mkdir -p /data/static
 
       - name: Restore the mail templates
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         id: mail-templates
         with:
           path: "src/backend/core/templates/mail"