don't skip cert check for an example & use HTTPS

This changes two URLs from http to https and it fixes a Dockerfile to
stop skipping certificate validation. It also adds the ca-certificates
package to that Dockerfile example.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
forbid chained onbuild, from & maintainer triggers

This changes the way onbuild works:
- forbids the chaining of onbuild instructions
- forbids the use of `onbuild from`
- forbids the use of `onbuild maintainer`

It also makes docker throw errors when encountering such triggers when
executing the triggers during `FROM`.

Three tests have been added:
- ensure that chained onbuild (`onbuild onbuild`) is forbidden
- ensure that `onbuild from` is forbidden
- ensure that `onbuild maintainer` is forbidden

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Fix build command auth by adding the auth config and registry objects to the build job environment.

Docker-DCO-1.1-Signed-off-by: Jake Moshenko <jake@devtable.com> (github: jakedt)
small update to API docs to go with #4276

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Merge pull request #4248 from unclejack/https_fixes

don't skip cert check for an example & use HTTPS
Fix cli argument usage typo for docker --mtu

Docker-DCO-1.1-Signed-off-by: Danny Berger <dpb587@gmail.com> (github: dpb587)
Merge pull request #4260 from unclejack/onbuild_fixes

forbid chained onbuild, from & maintainer triggers
Merge pull request #4271 from DevTable/fixbuildauth

Fix build auth by adding config to the build job environment
Add simple "grep" to hide the harmless "warning: no packages being tested depend on ..." in "go test -coverpkg ..." output

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4283 from tianon/hide-no-packages-being-tested

Add simple "grep" to hide the harmless "warning: no packages being tested depend on ..." in "go test -coverpkg ..." output
Drop -mtu from docs in favor of --mtu only

Docker-DCO-1.1-Signed-off-by: Danny Berger <dpb587@gmail.com> (github: dpb587)
Merge pull request #4277 from SvenDowideit/tianon-made-me-doit

small update to API docs to go with #4276
Merge pull request #4280 from dpb587/bugfix-mtu-typo

Fix argument usage typo for docker --mtu