BuildKit allows using alternative Dockerfile syntaxes to introduce new features
without having to update Docker itself. The general recommendation is to always
specify a "syntax" directive in a Dockerfile, so that (if needed) older versions
of Docker can download the correct syntax to build the Dockerfile.
This updates our examples to include a syntax directive, to make users more familiar
with these directives, and to illustrate best-pracitces in our documentation.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
