ps command doesn't work in docker container

ps is not installed in the base wheezy image. Try this from within the container:

RUN apt-get update && apt-get install -y procps

use docker top

docker top <container ID>

In case you can't install the procps package (don't have proper permissions) you can use /proc directory.

The first few directories (named as numbers) are PIDs of your processes. Inside directories, you can find additional information useful to decipher which process is connected to each PID. For example, you can use the cat command to view "cmdline" file to check which process is connected to PID.

$ ls /proc
1 10 11 ...

$ ls -1 /proc/22
attr
autogroup
auxv
cgroup
clear_refs
cmdline
...

$ cat /proc/22/cmdline 
/bin/sh