UID2-7335: upgrade libexpat (expat) to patch CVE-2026-45186

Adds the Alpine expat package to the apk --no-cache --upgrade step so
libexpat is bumped to 2.8.1-r0 at build time, clearing the HIGH-severity
DoS-via-crafted-XML finding (CVE-2026-45186). Bumping the pinned base
image SHA is insufficient: the latest 21-jre-alpine-3.23 image still
ships libexpat 2.7.4-r0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: swibi-ttd

Dockerfile

@@ -16,7 +16,7 @@ COPY ./target/${JAR_NAME}-${JAR_VERSION}-sources.jar /app
 COPY ./conf/default-config.json /app/conf/
 COPY ./conf/*.xml /app/conf/
 
-RUN apk add --no-cache --upgrade libpng libcrypto3 libssl3 musl musl-utils gnutls && adduser -D uid2-core && mkdir -p /app && chmod 705 -R /app && mkdir -p /app/file-uploads && chmod 777 -R /app/file-uploads && mkdir -p /app/pod_terminating && chmod 777 -R /app/pod_terminating
+RUN apk add --no-cache --upgrade libpng libcrypto3 libssl3 musl musl-utils gnutls expat && adduser -D uid2-core && mkdir -p /app && chmod 705 -R /app && mkdir -p /app/file-uploads && chmod 777 -R /app/file-uploads && mkdir -p /app/pod_terminating && chmod 777 -R /app/pod_terminating
 USER uid2-core
 
 CMD java \