Give a deprecation warning when using the apk-install wrapper

[pierreozoux]

No description provided.

[andyshinn]

We were originally going to do this in #18. I think this is probably fine. One request is that we should probably pipe the message to stderr like #18 did so it doesn't clobber normal output on stdout.

Ping @tianon. Is this something that we feel is fine for upstream?

[tianon]

If it goes to stderr, yeah, seems sane to me -- though in the alpine images, this script is not included, right?

[andyshinn]

Oh yea, you are right. It isn't even in library images. Disregard!