support/docker: limit the number of layers

The official documentation [0] suggests limiting the number of layers
generated from a dockerfile. A layer is created for each RUN (and COPY
and ADD) command. But we are only ever interested in the final image,
so the intermediate layers are useless to us.

Limit the number of RUN commands to limit the number of generated
layers.

[0] https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#minimize-the-number-of-layers



Reported-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>