Currently, the Docker Build and Push step does not support adding an expression for the entire Build Args field. You can set an expression in one of the argument's values but, not for the whole Build Args. For example, this works

buildArgs:

key: <+pipeline.variables.someValue>

But, we'd like the ability to provide a comma separated list of key/value pairs to this field like this instead,

bulidArgs: <+pipeline.variables.buildArgs>