field 22 of /proc/1/stat has the start time of the container (in jiffies)

field 22 of /proc/self/stat has the current time (in jiffies)

so...

$(( ( $(cut -d' ' -f22 /proc/self/stat) - $(cut -d' ' -f22 /proc/1/stat) ) / 100 ))

gives you the container uptime in seconds.