Launch the base docker that you want to customize.
$ sudo docker run -it ubuntu:latest
It may lead to a shell command. If the command does not lead to shell, you may grab the container id and launch a shell using the docker exec command.
$ sudo docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
278fb8fe9171 ubuntu:latest "/bin/bash" About an hour ago Up About an hour cranky_montalcini
$ sudo docker exec -it 278fb8fe9171 bash
Inside the container install python, and python libraries - flask and cassandra
root@278fb8fe9171:/# apt-get update -y && apt-get install -y python-pip python-dev build-essential
root@278fb8fe9171:/# pip install cassandra-driver flask
Exit from the container shell and run the following commands to create a custom image
$ sudo docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
278fb8fe9171 ubuntu:latest "/bin/bash" 14 minutes ago Up 14 minutes cranky_montalcini
$ sudo docker commit 278fb8fe9171 abasar/web_tier:1.0
sha256:d6a39aa114c8c09a7b50d864186aae082f3ea2d2d17b4368ff68e152b626dcf4
Optionally, you can push to docker hub to reuse later.
$ sudo docker push abasar/web_tier:1.0