add blogpost checker

[jo2]

Add blogpost-checker as merge gate.

See it in action: https://github.com/jo2/devblog/pull/1.

Once this is merged, we can add a branch protection on master which requires a successful run of pull-and-run-blogpost-checker-image.

[netlify[bot]]

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[acetous]

Funktioniert! :) Bitte mergen.