Add some information about building the docker image locally

[PromoFaux]

What does this PR aim to accomplish?:

Adds a slightly more fleshed out version of the instructions on the V6 readme. Includes some examples for users who are asked to checkout alternative branches

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code and I have tested my changes.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

---

• I have read the above and my PR is ready for review. Check this box to confirm

[netlify]

✅ Deploy Preview for pihole-docs ready!

Name	Link
🔨 Latest commit	424ddf6
🔍 Latest deploy log	https://app.netlify.com/sites/pihole-docs/deploys/66e2055342fe180008b3ebfd
😎 Deploy Preview	https://deploy-preview-1080--pihole-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.