docker: drop ineffective wrapper-jar exclusion from poms stage

The poms stage removed `.mvn/wrapper/maven-wrapper.jar` to try to keep the dependency-download layer stable across wrapper-jar changes. It didn't work: the build stage copies `.mvn/wrapper` (jar included) before the `COPY --from=poms` and the dependency `RUN` steps, so a wrapper-jar change still invalidates every later layer regardless of what poms emits.

Making the dependency layers independent of the wrapper jar would mean giving up the offline prefetch or restructuring how Maven is invoked, and the wrapper jar changes rarely enough that it isn't worth that complexity.

Revert the exclusion and correct the comments to describe the layer's actual cache inputs.

Author: guusdk

Dockerfile

@@ -6,10 +6,6 @@ WORKDIR /usr/src
 COPY . .
 # Wipe any files not called pom.xml or *.jar
 RUN find . -type f -and \! -name pom.xml -and \! -name '*.jar' -delete
-# Drop the Maven wrapper jar from the `poms` stage output: the build stage copies `.mvn/wrapper` separately.
-# Keeping it here is redundant, and would make the `poms` stage output change when the wrapper jar changes.
-# (That output is later copied into the dependency-download layer.)
-RUN rm -f .mvn/wrapper/maven-wrapper.jar
 # Clear up any (now) empty diretories
 RUN find . -type d -empty -delete
 
@@ -23,8 +19,8 @@ RUN mkdir -p .mvn
 COPY .mvn/wrapper .mvn/wrapper
 
 # First, copy in the pom.xml files (and any checked-in jars) and fetch the dependencies into a real image layer.
-# Because this layer depends on the POMs (and the Maven wrapper scripts/config), it stays stable until one of those
-# inputs changes, so dependency downloads are reused whenever the source changes but the dependencies don't.
+# This layer depends on the POMs, the checked-in jars, and the Maven wrapper (copied above), so it stays cached
+# until one of those changes; dependency downloads are reused whenever the source changes but those inputs don't.
 COPY --from=poms /usr/src/ .
 # I don't know why we need all three either.
 RUN ./mvnw -e -B dependency:resolve-plugins -Dmaven.test.skip -Dmaven.repo.local=/tmp/m2_repo
@@ -36,7 +32,7 @@ RUN ./mvnw -e -B dependency:get -DgroupId=org.codehaus.plexus -DartifactId=plexu
     ./mvnw -e -B dependency:get -DgroupId=org.junit -DartifactId=junit-bom -Dpackaging=pom -Dversion=5.13.4 -Dmaven.repo.local=/tmp/m2_repo && \
     ./mvnw -e -B dependency:get -DgroupId=org.mockito -DartifactId=mockito-bom -Dpackaging=pom -Dversion=5.22.0 -Dmaven.repo.local=/tmp/m2_repo
 
-# Above here is only affected by the pom.xml files, so the layer is stable.
+# Above here is affected only by the POMs, checked-in jars, and the Maven wrapper, so the layer is usually stable.
 
 # Now, copy in all the source, and actually build it, skipping the tests.
 # The offline build reads the /tmp/m2_repo that the layers above populated.