HTTP_PROXY support

[rarkins]

We want all tools to be able to support HTTP_PROXY, including with a wrapper if necessary. We also want to support that a buildpack-based Dockerfile can be built behind a HTTP_PROXY.

[rarkins]

To properly test proxy, we need a container running without any direct internet access.

This seems relevant: https://forums.docker.com/t/internal-network-between-containers-without-external-network-access/41751/9

Found via https://stackoverflow.com/questions/39913757/restrict-internet-access-docker-container

In theory there could be different proxies during build and during deployment so need to think how that would work. Our ideal is that it simply needs HTTP_PROXY passed in, e.g.

Build: docker build --build-arg HTTP_PROXY=http://10.0.0.1 . -t my_image

Run: docker up -d -e HTTP_PROXY=https://proxy.company.com my_image

We can address it this way:

• For tools (build or runtime) which respect HTTP_PROXY then we do nothing additional
• For tools which can use a proxy but need it provided a different way (e.g. config file, or CLI arg) then we perhaps need to wrap their CLI command to prepare the proxy config before calling the actual command
• For tools which don't support proxy, we need to find alternatives or document that it's impossible. Ideally zero of those in build.

[rarkins]

We probably need to think about DNS too. Maybe the proxy container will also be the DNS forwarder.

[viceice]

at least apt needs a special handling on install, which we support via APT_HTTP_PROXY env. thats already tested well. also ci tests are using the apt proxy