Docker image: Upgrade base image to UBI9 & Switch to runtime-only Ope…

…nJDK (#75)

Signed-off-by: Florian Hotze <dev@florianhotze.com>

src/main/docker/Dockerfile.jvm

@@ -26,7 +26,8 @@
 # This scripts computes the command line to execute your Java application, and
 # includes memory/GC tuning.
 # You can configure the behavior using the following environment properties:
-# - JAVA_OPTS: JVM options passed to the `java` command (example: "-verbose:class")
+# - JAVA_OPTS: JVM options passed to the `java` command (example: "-verbose:class") - Be aware that this will override
+#   the default JVM options, use `JAVA_OPTS_APPEND` to append options
 # - JAVA_OPTS_APPEND: User specified Java options to be appended to generated options
 #   in JAVA_OPTS (example: "-Dsome.property=foo")
 # - JAVA_MAX_MEM_RATIO: Is used when no `-Xmx` option is given in JAVA_OPTS. This is
@@ -77,7 +78,8 @@
 #   accessed directly. (example: "foo.example.com,bar.example.com")
 #
 ###
-FROM registry.access.redhat.com/ubi8/openjdk-21:1.20
+# Use Red Hat Universal Base Image 9 with runtime-only OpenJDK 21 as base. For Builder images, use ubi9/openjdk-21 instead.
+FROM registry.access.redhat.com/ubi9/openjdk-21-runtime:1.21
 
 ARG BUILD_DATE
 ARG VCS_REF