use alpine:3.2 in the docker image

[arschles]

currently using ubuntu-debootstrap:14.04

[arschles]

requires minio/mc#1533 to get an alpine compatible mc binary

[arschles]

Note that minio/mc#1533 was superseded by minio/mc#1539. Now that it's in, the DOCKERIMAGE=1 environment variable must be set to get the server and mc to run even when they were built without cgo. The var is set for both images in #32.

The server will still need to be built with CGO_ENABLED=0 to make this work. That's tracked at minio/minio#1033

[mboersma]

Workflow made a decision to move to ubuntu-slim (via quay.io/deis/base) as a Docker image base for security reasons (mostly that CVEs are patched more quickly, and that quay.io can scan for vulnerabilities). So I think this is moot after #98 and I'm closing it.