Add simple Alpine-based variant

[tianon]

No description provided.

[yosifkit]

I built this version that uses gosu and the duplicate that uses su-exec and the size difference is only minor:

$ docker images redis
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
redis               3-alpine-gosu       574ba9032328        12 seconds ago      15.94 MB
redis               3-alpine-su         ccab53ee1294        3 seconds ago       13.52 MB

I'd rather stick with the known gosu, since it is already prevalent in the official images and has versioned releases for easy upgrade on bugs.

[ncopa]

i wrote su-exec because I thought gosu was unnecessarily bloated ;) gosu is bigger than busybox and apk-tools together. I would prefer to use su-exec in the alpine images if possible.

[tianon]

I'm not opposed to switching to su-exec, but for the official images, consistency is much more important than bloat. If su-exec can be shown to be easily installable in all the cases where gosu currently is (pre-compiled static binaries are available for a large number of architectures), and can be shown to be completely feature equivalent (since gosu was written in Go specifically to be able to leverage the exact code Docker itself uses for parsing user:group strings), then we can evaluate switching.

[ncopa]

su-exec is completely feature equivalent. It handles all the user:groupcases mentioned in
https://github.com/opencontainers/runc/blob/master/libcontainer/user/user.go#L230 I did test all the possible cases I could think of and compared with gosu.

Not sure how far it make sense to pull this consistency thing. I mean, we don't port apt-get to alpine just to be consistent. The gosu tool was made because the tools provided on debian systems didn't do what we need it to. Ideally that tool should be upstreamed so you can apt-get install gosu. On alpine you can apk add su-exec for the exact same functionality.

[wol4max]

I agree with @ncopa on this. One of the key goals of Alpine is to reduce bloat. And introducing list specific optimizations is perfectly OK. My vote is to go with su-exec.