Update docker/build-push-action action to v6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
docker/build-push-action	action	major	v5 -> v6

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Release Notes

docker/build-push-action (docker/build-push-action)

v6

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[andyundso]

@justinclift it seems that this major update just adds support for build summaries:
https://github.com/docker/build-push-action/releases/tag/v6.0.0

not sure if we need it, but they would be available here:
https://github.com/pgautoupgrade/docker-pgautoupgrade/actions/runs/10985508491?pr=45

[justinclift]

Thanks @andyundso. I reckon they look potentially useful, and it doesn't seem like it should mess anything up.

Lets merge it and see how it goes. 😄

[justinclift]

On that note, I've just pushed some commits to fix some trivial CI warnings about Dockerfile syntax.

Didn't realise it was a problem we should fix until seeing that summary. 😉

So, the summary should look a bunch cleaner now. 😁

[andyundso]

@justinclift very nice, thank you very much!