Follow-up from "Add BuildKit documentation as secure alternative to Kaniko"

The following discussion from !192429 (merged) should be addressed:

• @marcel.amirault started a discussion: (+2 comments)

  This example is important, but there are limitations to CI/CD variables that mean we try to avoid calling them a "secure" option. We'd also need to talk about variable masking, protected variables (potentially).

  For example, seeing echo "$BUILD_SECRET" in an example always makes me 🙈

  Do you think we could remove this section for now, and add it in a followup that I can get reviewed by folks in pipeline security? There's already a TON of great examples in here, so I think we can delay this one section for a followup? 🙏

Link for reference: !192429 (30619be6)