Revert the base docker image back to JDK 11

JDK 21 requires that sbt is of version 1.9.0 or greater. This poses problems to some projects. As the other changes to support a multi-platform docker image had flaws too, this change shouldn't be a problem.

Author: mzuehlke

build.sbt

@@ -345,7 +345,7 @@ lazy val metadataSettings = Def.settings(
 
 lazy val dockerSettings = Def.settings(
   dockerBaseImage := Option(System.getenv("DOCKER_BASE_IMAGE"))
-    .getOrElse("eclipse-temurin:21-alpine"),
+    .getOrElse("eclipse-temurin:11-alpine"),
   dockerCommands ++= {
     val curl = "curl -fL --output"
     val binDir = "/usr/local/bin"