ci: support static cross-compilation with vendored UCI

Add Dockerfile.ci with pkg-config for vendored UCI builds. Switch
Jenkinsfile from pre-built Docker images to Dockerfile-based agents
for full control over the build environment. Add static CRT linking
via RUSTFLAGS for standalone static binaries across the CI matrix.

Dockerfile.ci

@@ -0,0 +1,7 @@
+ARG TARGET
+
+FROM ghcr.io/rust-cross/rust-musl-cross:${TARGET}
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends pkg-config \
+    && rm -rf /var/lib/apt/lists/*