be a little more explicit about what WORKDIR does

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Always download things over https

Docker-DCO-1.1-Signed-off-by: Joe Shaw <joe@joeshaw.org> (github: joeshaw)
Merge pull request #3938 from SvenDowideit/workdir-doc-tweak

be a little more explicit about what WORKDIR does
Merge pull request #3944 from joeshaw/patch-1

Always download things over https