feat: harden reviewed registry releases

ci/Dockerfile

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+# By Nic Weyand! Pinned toolchain image for the repository-scoped isolated runner.
+FROM rust:1.98.0-trixie@sha256:620dbcd124499c59e2406d3741574b5c5838cf9eb9656f0c3a03948f79b02959
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+        cmake \
+        openssh-client \
+        python3 \
+        shellcheck \
+    && rm -rf /var/lib/apt/lists/* \
+    && rustup component add clippy rustfmt
+
+WORKDIR /workspace

ci/README.md

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+# Isolated Forgejo runner image
+
+Build the reviewed pinned image and register a repository-scoped runner with only
+the container label below:
+
+```bash
+docker build --pull -t argand-site-registry-ci:0.2.0 -f ci/Dockerfile .
+forgejo-runner register --no-interactive \
+  --instance https://git.argand.org \
+  --token REPOSITORY_REGISTRATION_TOKEN \
+  --name argand-site-registry-isolated \
+  --labels site-registry-isolated:docker://argand-site-registry-ci:0.2.0
+```
+
+Run the daemon with capacity one, no host label, no bind-volume allowlist, no
+Docker socket inside jobs, `privileged: false`, and container limits of two CPUs,
+4 GiB memory and 4 GiB memory plus swap. The workflow fetches the exact public
+commit without repository credentials. This runner contains no dataset, reviewer,
+release-signing or activation authority.