docs: 01-intro.md: add missing . in Dockerfile examples
docs: 01-intro.md: add missing . in Dockerfile examples

The . is required, otherwise user will get syntax error.

Signed-off-by: Douglas Schilling Landgraf <cb5f9630225b7afa1e13760d30868c8ce876fc60@redhat.com>

projectatomic/buildah,projectatomic/buildah,nalind/buildah,nalind/buildah-1,projectatomic/buildah,nalind/buildah-1,nalind/buildah-1,rhatdan/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,nalind/buildah,TomSweeneyRedHat/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah