implement cross-arch image naming convention for 'blackbox' actions

[glikson]

For blackbox actions, we need to pull the image from docker hub as specified in the action metadata. This is not going to work if the hardware architecture of the Docker host is different than the one that the particular image was built for (e.g., ARM vs x86_64). Given that Docker doesn't seem to have a proper support for this (yet?), we can use tags or suffixes to enable maintaining different versions of the image for difference kinds of architecture (e.g., "myappimage-x86_64" vs "myappimage-arm"), and dynamically pull the one matching the target architecture (e.g., based on uname -i).