You don't necessarily need containers to get this to work (containers are generally only useful if you have multiple object instances).
The best way is to use image points. If you don't know how image points work, the Ghost Shooter tutorial explains them. Basically if you place an image point where you want your object to be positioned on your player, then you can use the event 'Always' - Gun: 'Set position to another object'. You can position it by the player, image point 1, and it goes to where you placed the image point in the picture editor.