support/docker: run apt-get update and apt-get install in two RUNs

In commit 7517aef4d (support/docker: limit the number of layers),
we reduced the number of layers by coalescing multiple RUN commands
into less commands.

In doing so, we especially coalesced "apt-get update" with "apt-get
install".

However, the distribution we used is a pinned version of stretch, so
we know that running apt-get update will always yield the same apt
database.

If we split the two apt-get commands, then we can re-use any local
intermediate image when we need to update the list of packages to
install; this helps quite a bit when testing the docker files over
and over again, with just slight variants in the packages list.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

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