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/*

Jenkinsfile

@@ -12,9 +12,9 @@ pipeline {
 				}
 
                 agent {
-                    docker {
-                        image "ghcr.io/rust-cross/rust-musl-cross:${TARGET}"
-						alwaysPull true
+                    dockerfile {
+                        filename 'Dockerfile.ci'
+                        additionalBuildArgs "--pull --build-arg TARGET=${TARGET}"
                     }
                 }
 
@@ -44,6 +44,9 @@ pipeline {
 								echo "----------------------------------"
 
 								# 5. Check Toolchain & Build
+								# Force standalone static artifacts across the entire CI matrix.
+								export RUSTFLAGS="${RUSTFLAGS:+$RUSTFLAGS }-C target-feature=+crt-static"
+
                                 # We grep for "nightly" in the version string (e.g., "rustc 1.95.0-nightly...")
                                 if rustc -V | grep -q "nightly"; then
                                     echo "Detected Nightly toolchain. Enabling '-Z build-std' to support panic=abort..."