Use alpine and build docker image from source

[frosforever]

Connects to #51

• Modify Dockerfile to leverage smaller alpine based instance
• Build image from sources rather than downloading from pip

Automated builds on https://hub.docker.com/r/frosforever/saws/ building off docker branch can be run via:

docker run -it -v ~/.aws/:/root/.aws:ro frosforever/saws:docker

[donnemartin]

@frosforever thanks for the feedback on https://github.com/donnemartin/saws/issues/51, and for submitting a followup PR.

@sysadmiral @cliveza @stilvoid any interest in reviewing this PR?