netstat or alternative in docker ubuntu server 16.04 container

As you can see from https://packages.ubuntu.com/xenial/amd64/net-tools/filelist, netstat is provided by the net-tools package. Therefore, you can get netstat by running:

sudo apt update
sudo apt install net-tools

net-tools is probably not installed by default in the Docker image for Ubuntu 16.04 to keep the image size as small as possible.

In my opinion is better to begin adapting yourself to new tools.

ss is auto-defined as "another utility to investigate sockets"

ss -ltu

Will show listening sockets for tcp and udp.

For more information you can use ss --help or man ss. The program ss can be found in the iproute2 package.