Update to eclipse-temurin:21 and remove appuser (antlr#4675)

It seems this appuser wasn't being used at all, and the USER line wasn't
being set to enable it. In addition, the adduser utility didn't exist in
the old or updated version of temurin. This seems simpler and works.

Signed-off-by: James <james.henderson@dataductus.com>

docker/Dockerfile

@@ -1,4 +1,4 @@
-FROM eclipse-temurin:11 AS builder
+FROM eclipse-temurin:21 AS builder
 
 WORKDIR /opt/antlr4
 
@@ -14,23 +14,8 @@ RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install maven git -
     && mvn -DskipTests install --projects tool --also-make \
     && mv ./tool/target/antlr4-*-complete.jar antlr4-tool.jar
 
-FROM eclipse-temurin:11-jre
-
-ARG user=appuser
-ARG group=appuser
-ARG uid=1000
-ARG gid=1000
-
-RUN adduser \
-    --disabled-password \
-    --gecos "" \
-    --home "$(pwd)" \
-    --no-create-home \
-    --uid "${uid}" \
-    "${user}"
+FROM eclipse-temurin:21-jre
 
 COPY --from=builder /opt/antlr4/antlr4/antlr4-tool.jar /usr/local/lib/
 WORKDIR /work
 ENTRYPOINT ["java", "-Xmx500M", "-cp", "/usr/local/lib/antlr4-tool.jar", "org.antlr.v4.Tool"]
-
-