dockerfiles/README.md: Clarify update is required
dockerfiles/README.md: Clarify update is required

Tweak the dockerfiles/README.md file to make it clear that you MUST
update .circleci/config.yml when you update a docker image.  If you
don't update the CircleCI configuration, then the system will continue
to use the old image. That's as intended; the whole point of pinning is
to only use the image specifically identified.

This is a side-effect of pinning image dependencies.

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>

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