Changes to techdocs image which breaks validation should be detected on the PR #173
Description
When the techdocs image updates, there are no changes to MARKDOWN_FILES and hence validation is not run. This causes the image to be updated even though it may cause validation issues (for example, if markdownlint is updated). So, techdocs validation should be run on all files in case of image changes.
For example, this would have failed if it had tried to validate all docs: https://github.com/coopnorge/cloud-platform/pull/183