Don't build caches into Docker images

[mboersma]

Adds flags to pip and apk commands in the Dockerfiles to prevent those tools from creating cache data which gets built into the Docker images.

YMMV in terms of space savings, but not caching these items is generally considered a best practice for containers.

$ git checkout master && docker build -t azure-cli:before .
$ git checkout dont-cache-inside-docker-image && docker build -t azure-cli:after .
$ docker images
REPOSITORY         TAG       IMAGE ID            CREATED             SIZE
azure-cli          before    408d8e5affc2        3 days ago          500MB
azure-cli          after     285a1d5f2499        3 days ago          482MB

[msftclas]

@mboersma,
Thanks for your contribution as a Microsoft full-time employee or intern. You do not need to sign a CLA.
Thanks,
Microsoft Pull Request Bot

[troydai]

Looks good to me. @derekbekoe?