Make base image configurable

[phlax]

this PR allows the base image to be set with a BUILD_FROM env var

it also defaults to the more lightweight stretch-slim, and makes package install non-interactive

[ehough]

Love it. Thanks for your contribution!

One nitpicky issue before I merge; it seems that ENV DEBIAN_FRONTEND=noninteractive in the Dockerfile is not considered best practice. If you read through that issue, the consensus is to either omit the line entirely or instead use:

ARG DEBIAN_FRONTEND=noninteractive

so that the variable doesn't persist into the running container. Thoughts? I'm happy to make this tweak after merging, but wanted to give you an opportunity to opine and/or add another commit.

Thanks again!

[phlax]

hi @ehough - using an ARG seems like a good compromise - its needed for the slim image as it has no readline support

feel free to hack/squash/merge as you see fit

[ehough]

@phlax thanks again, and please feel free to send along any other suggestions or improvements.

[phlax]

@ehough ive used your build in a travis test:

https://travis-ci.org/phlax/nfs-client/builds/377456887

for an nfs-client im working on:

https://github.com/phlax/nfs-client

it actually uses my build - but only as the image is smaller

if its helpful i can PR to add a travis test using my client, optionally that pushes master to dockerhub on successful build

[ehough]

Very slick! That's a great idea - I would love to use your client for CI purposes. Please feel free to send along a PR and we'll get it merged. Once that foundation is in place, I can think of a bunch of scenarios that would be good to test. i.e. NFS v3, NFS v4, Kerberos, UDP/TCP, etc.

Thanks again.