UB1250 enclosed charging?

[Jack.Straw]

Hi. I purchased a UB1250 12v, 5Ah, SLA deep cycle battery. I intended to use it inside of a 5 liter mini-keg to power an 12v amplifier, as well as some 5v accessories (Bluetooth audio receiver & occasional phone charging). I just noticed that it says on the battery "do not charge in a sealed container". Can anyone explain this to me? My "sealed container" is roughly 6x the size of the battery... is this going to be a problem? This is my container:

Thanks for any advice you can offer!