Is atmospheric pressure acting only on the contents or also on the container?
According to the way this question is worded, adding 1 atmosphere would be incorrect. The reason is that they specify an empty container - if you have a rigid container and it is truly empty (high vacuum), then the absolute pressure would be zero, and the gauge pressure would be -1 atm (see this wikipedia article for reference to these terms).

Once the $\ce{CO2}$ sublimates, the final absolute pressure would be given by $PV=nRT$ - there is no need to add atmospheric pressure. To find the gauge pressure, you would actually subtract one atmosphere.

It is possible that the question wants you to assume that "empty" means "filled with air at 1 atm," but that is a guess and I would argue that it's not safe to assume that based on the way the question is written.