Add multiple-platform builds. Add experimental arm64 build.

docs/build.md

@@ -9,6 +9,8 @@
 ## Binaries
 ### Requirements
 - JDK 1.8
+- OpenJDK 11
+- OpenJDK 14
 
 ### Build
 ```bash
@@ -70,5 +72,13 @@ Then follow the instruction in the [Debian package](install/debian.md) document.
 ```
 Then follow the instructions in the [Docker install](install/docker.md#configure) document.
 
+### Multi-platform builds
+
+Provided with experimental docker feature [buildx](https://docs.docker.com/buildx/working-with-buildx/)
+To build the arm64 and amd64 images run:
+```bash
+./gradlew dockerBuildX
+```
+
 ## Next steps
 - [Integrate with your infrastructure](getting-started.md#integrate)