Various improvements to Dockerfiles

[jnoordsij]

This PR introduces the following three improvements to all Dockerfiles:

• Update from Alpine 3.19 to 3.21; given that the Docker official images only support the last two Alpine versions, this ensures up-to-date images are used
• Use consistent FROM + AS keyword casing to solve Docker style warning (see e.g. https://github.com/kubernetes/dashboard/actions/runs/13896030290/job/38876759635#step:8:10851 and https://docs.docker.com/reference/build-checks/from-as-casing/)
• Update a specific Node image tag with a major-only tag, to ensure the latest minor+patch version is used

Note these are practically unrelated changes, but given they all alter the same lines of code, I've opted to combine them into a single PR regardless. If any of these changes should be removed our submitted individually, let me know!

[k8s-ci-robot]

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