docs: add docs tutorial on custom policies with rego

[AnaisUrlichs]

Added tutorial on setting up a custom policy for your Dockerfile using Rego

[itaysk]

great thanks. we refer to policies and checks interchangeably, just wanna make sure it's intentional

[simar7]

great thanks. we refer to policies and checks interchangeably, just wanna make sure it's intentional

We last agreed on calling everything a check #5609

[AnaisUrlichs]

@simar7 are we still planning to rename the flag --policy?
I made sure I referred to it everywhere as a check instead of a policy

[simar7]

@simar7 are we still planning to rename the flag --policy?
I made sure I referred to it everywhere as a check instead of a policy

We should still support that for backwards compatibility but should add a newer flag --check and show a deprecation message for the old one --policy.

[simar7]

Left one comment but lgtm!