Allow end user to add additional packages to the image

[PromoFaux]

What does this PR aim to accomplish?:

Heavily inspired by Netdata's container. (Thanks @dschaper for the pointer)

Some people like nano, some people like vim. Why force them to use the better of the two? ;)

Also, anyone can apk add inside the container - this just makes it easier for those of us that always like to have whatever additional tools we need inside the container for debugging

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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)

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[yubiuser]

Documentation required.