bug#50432: DF shows incorrect disk usage which include disk usage of a mounted path

[Paul Eggert]

It sounds like df is reporting disk usage for the file system you're 
running your docker instance atop. In that case, there may be other uses 
of the file system, aside from the docker instance, which means df is 
reporting the correct numbers (and du is, too).

The problem, if it is a problem, is not likely to be in df itself; it's 
likely to be in the underlying filesystem code. You can check for this 
by running 'strace df' and looking at what numbers the system calls return.