Thank you for your post.
At this time, interactive Container fields in portals are not supported. This is documented in the FileMaker Pro 12 Help:
Home -> Designing and creating databases -> Editing objects on layouts -> Formatting and setting up field objects in Layout mode -> Specifying formats for container fields
Another customer on the forum also reported this issue, and you can find it at:
One possible workaround is to make the Container field a button in the portal, so when clicked, use the Go to Related Record script step and displays the Container field in another layout.
Let me know if you need additional clarification.
A script can also display the contents of the portal container in a container field on the same layout, but outside the portal and thus can be set as interactive.