Create sandbox for image build step

[nuest]

When building the image, the process should not download abitrary files but ideally only whitelisted online software repositories, such as Debian and CRAN (or MRAN).

Can this be realized by running the build within a specific Docker Network?

Related to #33

Some resources:

• https://steelmon.wordpress.com/2009/11/22/setting-up-a-strict-whitelist-proxy-server-using-squid/
• https://serverfault.com/questions/734526/whitelisting-outgoing-traffic-from-docker-containers
• https://security.stackexchange.com/questions/107850/docker-as-a-sandbox-for-untrusted-code
• https://hub.docker.com/r/sameersbn/squid/ (there are other images!)