Move notes about required buildkit

[matletix]

- What I did

The docker build command silently ignores the --output flag when buildkit is not used (see #2680 and #2736). It would help to make it clear from the begining of the documentation that this feature requires buildkit.

- How I did it

I just moved the note specifying the need for buildkit from the end to the begining of the related section in the documentation. I also did the same for the section about external cache sources.

- How to verify it

Check the diff

- Description for the changelog

Minor update to the CLI docker build documentation

[thaJeztah]

Thank you for contributing! It appears your commit message is missing a DCO sign-off,
causing the DCO check to fail.

We require all commit messages to have a Signed-off-by line with your name
and e-mail (see "Sign your work"
in the CONTRIBUTING.md in this repository), which looks something like:

Signed-off-by: YourFirsName YourLastName <yourname@example.org>

There is no need to open a new pull request, but to fix this (and make CI pass),
you need to amend the commit(s) in this pull request, and "force push" the amended
commit.

Unfortunately, it's not possible to do so through GitHub's web UI, so this needs
to be done through the git commandline.

You can find some instructions in the output of the DCO check (which can be found
in the "checks" tab on this pull request), as well as in the Moby contributing guide.

Steps to do so "roughly" come down to:

1. Set your name and e-mail in git's configuration:

   git config --global user.name "YourFirstName YourLastName"
   git config --global user.email "yourname@example.org"

   (Make sure to use your real name (not your GitHub username/handle) and e-mail)

2. Clone your fork locally

3. Check out the branch associated with this pull request

4. Sign-off and amend the existing commit(s)

   git commit --amend --no-edit --signoff

   If your pull request contains multiple commits, either squash the commits (if
   needed) or sign-off each individual commit.

5. Force push your branch to GitHub (using the --force or --force-with-lease flags) to update the pull request.

Sorry for the hassle (I wish GitHub would make this a bit easier to do), and let me know if you need help or more detailed instructions!

[matletix]

Thank you, should be good now :)

[thaJeztah]

awesome, thank you!

LGTM

[codecov-commenter]

Codecov Report

Merging #3730 (f41cbf9) into master (e1f24d3) will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## master #3730 +/- ##
=======================================
Coverage 59.15% 59.15% =======================================
Files 289 289 Lines 24674 24674 =======================================
Hits 14597 14597 Misses 9206 9206 Partials 871 871