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. Push the image to Docker hub.
6. 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:
docker build -t yourdockerusername/imagename .

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

6 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