feat(docker): add labels, pin alpine tag to the latest version

[NINNiT]

Just a few minimal Dockerfile improvements.

---

Adds the following standardized Dockerfile Labels:

LABEL org.opencontainers.image.source="https://github.com/maplibre/martin" 
LABEL org.opencontainers.image.licenses="Apache-2.0 OR MIT"

I've noticed that some dependency upgrade tools (renovate, dependabot, ...) can't fetch the martin changelog when creating Bump MRs. org.opencontainers.image.source should alleviate that..
Providing the licenses also doesn't hurt.

---

Furthermore I've pinned the tag to latest, which makes it a bit cleaner. (same behavior as not pinning the tag at all).
Pinning to a fixed version would be better - but without some sort of automation, keeping the tag up-to-date can be a hassle.

Seems like there are no problems regarding CI image caching however, as martin is already using the newest alpine version:

docker run --entrypoint /bin/sh ghcr.io/maplibre/martin:v0.16.0 -c "cat /etc/os-release""

NAME="Alpine Linux"
ID=alpine
VERSION_ID=3.21.3
PRETTY_NAME="Alpine Linux v3.21"
HOME_URL="https://alpinelinux.org/"
BUG_REPORT_URL="https://gitlab.alpinelinux.org/alpine/aports/-/issues"

[Copilot]

Pull Request Overview

This PR introduces two minimal improvements to the Dockerfile by standardizing OCI labels and pinning the base image tag.

• Updates the FROM instruction to use alpine:latest.
• Adds standardized labels for image source and licenses.

[CommanderStorm]

we have automatic dependency updates on
=> we get notified if something here is breaking => pinning to minor is imo fine for this one ^^

[CommanderStorm]

Thanks for the improvements

[CommanderStorm]

Actually, I am not sure if we use these labels or completely ignore them:

martin/.github/workflows/ci.yml

Lines 233 to 247 in c833034

	- name: Docker meta
	id: docker_meta
	uses: docker/metadata-action@v5
	# https://github.com/docker/metadata-action
	with:
	images: ghcr.io/${{ github.repository }}
	- name: Push the Docker image
	if: github.event_name != 'pull_request'
	uses: docker/build-push-action@v6
	with:
	context: .
	file: .github/files/multi-platform.Dockerfile
	push: true
	tags: ${{ steps.docker_meta.outputs.tags }}
	labels: ${{ steps.docker_meta.outputs.labels }}

Will have to investigate

[CommanderStorm]

(simplest way to investigate is merging and looking at the :main image)