docker: Adds support for alpine as base image

[kenjones-cisco]

Adds a variant for NSQ leveraging alpine as the base image.

Alpine is equally as small as busybox (~5 MB) but has more features
including the ability to install other packages.

[kenjones-cisco]

busybox is unfortunately very limited as there is no option for curl and wget is all but broken. So the ability to extend the base nsqio/nsq image is painful. This provides an equally simple base image that leverages busybox binaries internally while providing the option for more if needed.

Many of the major official images within Docker Hub are now including an alpine variant or using alpine exclusively.

To maintain backwards compatibility with what existed, I just added a separate docker file and updated the dist to build a new image with a different tag indicating alpine.

[mreiferson]

I would be amenable to switching the official image over to alpine, what do you think @jehiah?

cc @ploxiln

[jehiah]

sounds good to me

[kenjones-cisco]

Should I update the PR to replace the existing Dockerfile then?

[mreiferson]

Yes please!

[mreiferson]

We can't drop the symlinks for backwards compat

[mreiferson]

great, thanks!

Do you mind squashing this down to one commit?

[kenjones-cisco]

@mreiferson done

[mreiferson]

LGTM