Set VERSION build arg to image tag

This impacts the DEFAULT_DOCKCROSS_IMAGE in common.label-and-env which
determines the default image version used by the dockcross script. We
want the same tagged version of the image to be used its generated dockcross script (not `latest`).

tools/dockcross-command.sh

@@ -4,6 +4,9 @@ if (( $# >= 2 )); then
     image_complet=$1
     image=${image_complet%:*}
     tag=${image_complet#*:}
+    if test $tag = $image; then
+        tag="latest"
+    fi
     shift 1
 
     command=$*
@@ -17,7 +20,7 @@ if (( $# >= 2 )); then
     chmod +x ./dockcross-"$image"
     
     echo "Run command in dockcross-$image"
-    ./dockcross-"$image" $command
+    ./dockcross-"$image" -i dockcross/"$image:$tag" $command
 else
     echo "Usage: ${0##*/} <docker imag (ex: linux-x64/linux-x64-clang/linux-arm64/windows-shared-x64/windows-static-x64...)> <command>"
     exit 1