Document a repeatable container workflow for the Rust workspace

Add a checked-in Containerfile plus container-first documentation so Docker and Podman users have a canonical image build, bind-mount, and cargo test entrypoint. The README now links directly to the new guide.

Constraint: The repo already had runtime container detection but no checked-in Dockerfile, Containerfile, or devcontainer config
Rejected: Put all container steps inline in README only | harder to maintain and less reusable than a dedicated guide plus Containerfile
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep docs/container.md and Containerfile aligned whenever Rust workspace prerequisites change
Tested: docker build -t claw-code-dev-docs-verify -f Containerfile .
Tested: cargo test --workspace (host, in rust/)
Not-tested: Podman commands were documented but not executed in this environment
Not-tested: Repeated in-container cargo test --workspace currently trips crates/tools PowerShell stub detection on this minimal image even though host cargo test passes

Containerfile

@@ -0,0 +1,13 @@
+FROM rust:bookworm
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+        ca-certificates \
+        git \
+        libssl-dev \
+        pkg-config \
+    && rm -rf /var/lib/apt/lists/*
+
+ENV CARGO_TERM_COLOR=always
+WORKDIR /workspace
+CMD ["bash"]