I've noticed something weird I couldn't reproduce on other docker images such as ubuntu or on a "real" distribution (i.e: not running in a container engine).

When I launch the command: nc -l 9999, I expect to see the in /proc/net/tcp something like this:

0: 00000000:270F 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 132769 1 ffff880078ff4040 100 0 0 10 0

where the 00000000:270F part corresponds to <local_addr>:<local_port>. In this sample, I see 0.0.0.0:9999 (once converted to decimal). However on alpine docker I see random number for the port part instead of the 9999 I expect.

Here is a screenshot of my session: