ci: fix pipeline – remove container until arm64 image is rebuilt

The ci-cpp image was likely built for linux/amd64; the eulernest runner
is linux/arm64 (Raspberry Pi), so the container exited immediately
without running any steps (task 102 log: 10 lines, no step output).

Fix: run directly on the host runner and install cmake/g++ via apt-get
(same approach as the previously working workflow).

Also pin --platform=linux/arm64 in Dockerfile.ci-cpp so the next
image build produces the correct architecture. Once the image is pushed,
re-add the container: block to the workflow.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/docker/Dockerfile.ci-cpp

@@ -1,4 +1,4 @@
-FROM ubuntu:22.04
+FROM --platform=linux/arm64 ubuntu:22.04
 
 # Node.js 20 from NodeSource (Ubuntu Jammy ships v12 which is too old
 # for actions/checkout@v4 — static class blocks require Node.js >= 16).