uptime command gives weird results in a docker image

If I run a docker image and call uptime, I always get a longer value than the real one.

docker run -it 1d2dfb3a35ab
root@7efb3e947f73:/# uptime
 23:41:57 up 16 min,  0 users,  load average: 0.06, 0.02, 0.00

It hasn't been running for 16 minutes, not even 16 seconds. =:-O

Is this a known issue or what?

The displayed uptime is that of the host. The kernel does not track uptime for individual containers in this way (though Docker does).

The first process start time could be used as container start time:

stat /proc/1/cmdline

/proc/1/cmdline is a virtual file in the procfs, whose creation date is the same as the container creation date. The stat command shows its filesystem-level attributes, its creation date between them.