Simplify Gson setup (#6833)

projects/gson/Dockerfile

@@ -25,6 +25,4 @@ ENV MVN $SRC/maven/apache-maven-3.8.3/bin/mvn
 RUN git clone --depth 1 https://github.com/google/gson gson
 WORKDIR gson
 COPY build.sh $SRC/
-COPY pom.xml $SRC/gson/pom.xml
-COPY gson/pom.xml $SRC/gson/gson/pom.xml
 COPY *.java $SRC/

projects/gson/build.sh

@@ -15,8 +15,16 @@
 #
 ################################################################################
 
-MAVEN_ARGS="-Dmaven.test.skip=true -Djavac.src.version=11 -Djavac.target.version=11 -X"
-$MVN --batch-mode --update-snapshots verify ${MAVEN_ARGS}
+# Change Java version because Gson is currently compiled with Java 6, which is only
+# supported up to JDK 11, but Docker image uses newer JDK
+# Additionally choose >= Java 9 because tests won't compile otherwise (JDK-7196160)
+# Skip ProGuard because it is only needed for tests (which are skipped as well) and
+# because it would fail since `jmods` JDK folder is removed from this Docker image
+MAVEN_ARGS="-DskipTests -Dmaven.compiler.release=9 -Dproguard.skip -X"
+# Only build 'gson' Maven module
+cd gson
+$MVN --batch-mode --update-snapshots package ${MAVEN_ARGS}
+cd ..
 find ./gson -name "gson-*.jar" -exec mv {} $OUT/gson.jar \;
 
 ALL_JARS="gson.jar"