Add bash completion for `build --squash`

Signed-off-by: Harald Albers <github@albersweb.de>
(cherry picked from commit e91dbba7ff62a92bd395bc419f522fa66618ae7b)
Signed-off-by: Victor Vieux <vieux@docker.com>

contrib/completion/bash/docker

@@ -2006,6 +2006,7 @@ _docker_image_build() {
 		--quiet -q
 		--rm
 	"
+	__docker_is_experimental && boolean_options+="--squash"
 
 	local all_options="$options_with_args $boolean_options"