[docker] multiarch support: linux/amd64,linux/arm64,linux/arm/v7

make docker.buildx : build and push multiarch build.
(it can't be only build)

use buildx with the --cache-from and --cache-to options to cache the layers
(only the last built is cached)

Makefile

@@ -73,7 +73,7 @@ MANAGE += buildenv
 MANAGE += babel.compile
 MANAGE += data.all data.languages data.useragents
 MANAGE += docs.html docs.live docs.gh-pages docs.prebuild docs.clean
-MANAGE += docker.build docker.push
+MANAGE += docker.build docker.push docker.buildx
 MANAGE += gecko.driver
 MANAGE += node.env node.clean
 MANAGE += py.build py.clean