[CALCITE-3573] Upgrade to Gradle 6.0 container to build releases and javadoc
diff --git a/docker-compose.yml b/docker-compose.yml
index ef95550..fa69a87 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -16,7 +16,7 @@
 version: '3'
 services:
   dry-run:
-    image: gradle:5.6-jdk
+    image: gradle:6.0-jdk
     working_dir: /src
     command: sh -c "./docker.sh dry-run"
     network_mode: "host"
@@ -25,7 +25,7 @@
       - gradle-cache:/home/gradle/.gradle
 
   publish-release-for-voting:
-    image: gradle:5.6-jdk
+    image: gradle:6.0-jdk
     working_dir: /src
     command: sh -c "./docker.sh publish-release-for-voting"
     volumes:
@@ -33,7 +33,7 @@
       - gradle-cache:/home/gradle/.gradle
 
   clean:
-    image: gradle:5.6-jdk
+    image: gradle:6.0-jdk
     working_dir: /src
     command: sh -c "./docker.sh clean"
     volumes:
@@ -41,7 +41,7 @@
       - gradle-cache:/home/gradle/.gradle
 
   promote-release:
-    image: gradle:5.6-jdk
+    image: gradle:6.0-jdk
     working_dir: /src
     command: sh -c "./docker.sh promote-release"
     volumes:
@@ -49,7 +49,7 @@
       - gradle-cache:/home/gradle/.gradle
 
   test:
-    image: gradle:5.6-jdk
+    image: gradle:6.0-jdk
     working_dir: /src
     command: sh -c "./docker.sh test"
     volumes:
diff --git a/site/docker-compose.yml b/site/docker-compose.yml
index 2923492..6c163d8 100644
--- a/site/docker-compose.yml
+++ b/site/docker-compose.yml
@@ -28,13 +28,13 @@
     volumes:
       - .:/srv/jekyll
   generate-javadoc:
-    image: maven
+    image: gradle:6.0
     working_dir: /usr/src/calcite-avatica
-    command: sh -c "./gradlew javadocAggregate; rm -rf site/target/avatica/apidocs site/target/avatica/testapidocs; mkdir -p site/target/avatica; mv build/docs/javadocAggregate site/target/avatica"
+    command: sh -c "gradle javadocAggregate; rm -rf site/target/avatica/apidocs site/target/avatica/testapidocs; mkdir -p site/target/avatica; mv build/docs/javadocAggregate site/target/avatica"
     volumes:
       - ../:/usr/src/calcite-avatica
-      - maven-repo:/root/.m2
+      - gradle-cache:/home/gradle/.gradle
 volumes:
-  maven-repo:
+  gradle-cache:
 
 # End docker-compose.yml