[docker] upgrade gradle (no jdk16 support yet)

docker/gradle/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04 as downloader
+FROM ubuntu:20.04 as downloader
 
 ARG jdk=16
 ADD provision/jdk/java-${jdk}.sh provision/jdk/java-${jdk}.sh
@@ -7,11 +7,11 @@ RUN provision/jdk/java-${jdk}.sh
 ARG lombokjar=lombok.jar
 ADD https://projectlombok.org/downloads/${lombokjar} /lombok.jar
 
-ARG gradle=6.2.2
+ARG gradle=6.8.3
 ADD provision/gradle/gradle-${gradle}.sh provision/gradle/gradle-${gradle}.sh
 RUN provision/gradle/gradle-${gradle}.sh
 
-FROM ubuntu:18.04
+FROM ubuntu:20.04
 
 COPY --from=downloader /opt/gradle/ /opt/gradle/
 COPY --from=downloader /opt/jdk/ /opt/jdk/

docker/gradle/readme.md

@@ -6,7 +6,8 @@
 
 The gradle version to be used. Supported values:
 
-- `6.2.2` (default)
+- `6.8.3` (default)
+- `6.2.2`
 - `6.0.1`
 - `5.6`
 - `5.1.1`
@@ -19,21 +20,21 @@ The gradle version to be used. Supported values:
 (To be executed from the `<lombokhome>/docker` directory)
 
 ```
-docker build -t lombok-gradle-jdk13 -f gradle/Dockerfile .
+docker build -t lombok-gradle-jdk16 -f gradle/Dockerfile .
 
-docker build -t lombok-gradle-jdk13 --build-arg lombokjar=lombok-1.16.20.jar -f gradle/Dockerfile .
+docker build -t lombok-gradle-jdk16 --build-arg lombokjar=lombok-1.18.20.jar -f gradle/Dockerfile .
 ```
 
 ## Example run commands:
 
 ```
-docker run -it lombok-gradle-jdk13
+docker run -it lombok-gradle-jdk16
 
-docker run --rm -it -v /<lombokhome>/dist/lombok.jar:/workspace/classpath/lombok.jar lombok-gradle-jdk13
+docker run --rm -it -v /<lombokhome>/dist/lombok.jar:/workspace/classpath/lombok.jar lombok-gradle-jdk16
 ```
 
 ## Example container commands:
 
 ```
 gradle assemble
-```cd ...
+```

docker/provision/gradle/gradle-6.8.3.sh

@@ -0,0 +1,4 @@
+apt-get update && apt-get install -y wget unzip
+wget https://services.gradle.org/distributions/gradle-6.8.3-bin.zip -O gradle.zip
+mkdir /opt/gradle && unzip -d /opt/gradle gradle.zip
+mv /opt/gradle/gradle-6.8.3 /opt/gradle/gradle