uptime command gives weird results in a docker image

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.

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