bug 1468490: Update to Debian 9 (stretch) base
bug 1468490: Update to Debian 9 (stretch) base

Update the Python base image from Debian 8 (jessie) to 9 (stretch),
which is the current LTS release, along with some additional cleanup:

Move the ENV declarations to the top, to reduce the number of
intermediate images.

Move all the package installation to the same section, to avoid
refreshing the apt-get cache twice. Include the gpg package and
dependencies, which is no longer in the default image. Update the names
of the MySQL packages.

Update the node.js installation code to bump the minor version and use
the current code for importing GPG signing keys. Drop the unused yarn
install. The 8.x node.js update will come later.

mozilla/kuma,Elchi3/kuma,escattone/kuma,a2sheppy/kuma,a2sheppy/kuma,jwhitlock/kuma,escattone/kuma,Elchi3/kuma,jwhitlock/kuma,SphinxKnight/kuma,SphinxKnight/kuma,a2sheppy/kuma,SphinxKnight/kuma,a2sheppy/kuma,SphinxKnight/kuma,Elchi3/kuma,mozilla/kuma,Elchi3/kuma,jwhitlock/kuma,jwhitlock/kuma,escattone/kuma,Elchi3/kuma,SphinxKnight/kuma,SphinxKnight/kuma,mozilla/kuma,mozilla/kuma,jwhitlock/kuma,mozilla/kuma,a2sheppy/kuma