Update CONTRIBUTING.md Section 2.7.b with information about Docker

[roslynwythe]

Overview

We need to update Section 2.7.b with information about Docker, including instructions for keeping the local Docker image up to date by pulling updates from Dockerhub as needed.

Action Items

• Open CONTRIBUTING.md in your IDE
• See draft instructions at Keep local Docker images up to date #3488 (comment)
• Edit CONTRIBUTING.md to incorporate content from the draft, making changes as required so that the format and style are consistent with CONTRIBUTING.md, and making any changes required to improve clarity.
• Changes to CONTRIBUTING.md cannot be tested locally, rather they must be tested after pushing the issue branch to your fork of the repository. Push your issue branch in the usual manner, but before creating the Pull Request, check your updates using this test URL. Also store the test URL for use in a later step:

https://github.com/[REPLACE WITH GITHUB HANDLE]/website/blob/[REPLACE WITH NAME OF ISSUE BRANCH]/CONTRIBUTING.md

(for example: https://github.com/bonniewolfe/website/blob/issue-branch-1234/CONTRIBUTING.md)

• Create a pull request with your changes. In the Pull Request, after the "Why did you make the changes" section, add this line to help reviewers, replacing the text in brackets (and the brackets) with the test URL from the previous Action Item.

For Reviewers: Do not review changes locally, rather, review changes at [REPLACE WITH TEST URL]

Resources/Instructions

Draft changes #3488 (comment)
CONTRIBUTING.md
Getting started with writing and formatting on GitHub

[HackforLABot]

Hi @jenjenkayi, thank you for taking up this issue! Hfla appreciates you :)

Do let fellow developers know about your:-
i. Availability: (When are you available to work on the issue/answer questions other programmers might have about your issue?)
ii. ETA: (When do you expect this issue to be completed?)

You're awesome!

P.S. - You may not take up another issue until this issue gets merged (or closed). Thanks again :)

[jenjenkayi]

i. Availability: 10/23 2pm - 5pm
ii. ETA: 10/23 by 5pm