Added non-root user to docker images.

[arahja]

Added non-root user to docker images.
This gives you the ability to run ntfy as a non-root user. resolves #966

[binwiederhier]

Thanks

[binwiederhier]

I couldn't get this to build so I had to revert it. https://github.com/binwiederhier/ntfy/actions/runs/8195110634/job/22412872174

Feel free to open another PR with a fix for this. Try building via make release-snapshot to see if it would build o Github

[arahja]

Sounds good, I will rework this PR.

[arahja]

@binwiederhier I ran the make release-snapshot build command as suggested. I was able to reproduce locally. I worked around the issue by using the full path for the adduser command. There seems to be a slight variance between the linux/arm/v7/alpine and the linux/amd64/alpine images related to the "$PATH" variable. The new PR is #1052