fix: swap Chainguard JRE to BellSoft Liberica JRE 21

Chainguard free tier only offers :latest (currently JDK 26, unpinned);
the :openjdk-21 tag requires a paid subscription, breaking CI.

Switch both Dockerfiles to bellsoft/liberica-runtime-container:jre-21-slim-glibc:
- Pinned to JDK 21 LTS
- Smallest image (199 MB vs 441/491 MB)
- glibc-based Alpaquita Linux, sh-only (no bash, no pkg manager)
- Free, multi-arch (amd64 + arm64)
- Has sh — required by cameleer-server's DeploymentExecutor (withCmd "sh -c")

Use nobody:nobody (65534) instead of Chainguard's nonroot (65532).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/runtime-base/Dockerfile

@@ -1,7 +1,6 @@
-# Wolfi-based JRE, glibc, daily-rebuilt with near-zero baseline CVEs,
-# signed images + SBOM published, non-root by default. Pin by digest in
-# production overlays.
-FROM cgr.dev/chainguard/jre:openjdk-21
+# BellSoft Liberica JRE 21 on Alpaquita Linux (glibc, minimal, 199 MB).
+# Pin by digest in production overlays.
+FROM bellsoft/liberica-runtime-container:jre-21-slim-glibc
 
 WORKDIR /app
 
@@ -15,3 +14,4 @@ COPY cameleer-log-appender.jar /app/cameleer-log-appender.jar
 # -cp + main; native: exec) and overrides via withCmd("sh","-c",...).
 # Setting one here only creates drift between this image and the actual
 # runtime command.
+USER nobody