Finally, I found an article from G-Loaded Journal. In short, we need to make
mount treat the file as a block device. First, we check which one is free:
Afterwards, we set it as a block device:
losetup /dev/loop0 /path/to/container1
Other instructions are pretty straightforward.
Note: make sure you're not using
mkfs or similar commands on an existing encrypted volume! The instructions might be confusing on that point.
Note 2: It is always a good idea to back up a dumpfile before messing with it. Just
cp your file to someplace else before messing with it!