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Actually just did this with a friend. Face the cell with the Dahl chest and turn around, there is a ladder on the right hand side. Climb it and follow its path until you see a railing and three pipes going toward another room across from the landing you are on. Jump over the railing onto the three pipes and go into the room. There is some minor loot but you want the circuit breakers. Turn them all so they are red and all the cell doors will be opened. Jump down and enjoy the loot.
To open this chest, you need to continue through the main quest and go through Jacks 3 hurdles in the quest 'Where Angels Fear to Tread' After that the quest 'Bearer of Bad News' becomes available. Then when that is finished you are given access to Rolands Armory (the locked room)