There are some steps required to create your own docker image.

1. Have a Docker hub account, https://hub.docker.com/
2. Install docker.io package on 64 bit system. 
3. Create a Dockerfile. You will find one in this directory.
4. Build the image from Dockerfile.
5. Login on Docker hub
6. Push the image to Docker hub.
7. Specify the image in your bitbucket-pipelines.yml file

1 Docker hub account
Well, sign up here =) https://hub.docker.com/

2 Install docker
sudo apt-get install docker.io

3 Create Dockerfile
Find example in this directory.

4 Build docker image
In the same directory as you have your Dockerfile, run command:
sudo docker build -t yourdockerusername/imagename .

5 Login on Docker hub
Before publishing you must login on Docker hub
sudo docker login

6 Push image to Docker hub
Run command
sudo docker push yourdockerusername/imagename

7 Specify the image in your bitbucket-pipelines.yml file
As documented here: https://confluence.atlassian.com/display/BITBUCKET/Use+Docker+images+as+build+environments+in+Bitbucket+Pipelines

You can use your image on a top level as well as on a specific step like this. 
image: yourdockerusername/imagename
