README: clarify docker image tags
README: clarify docker image tags

Some people are confused by `latest` versus `v3`, so clarify that.
Also document that we publish releases and Alpine variants.

While here, remove the reference to the alternative Docker image,
as these days it's somewhat redundant (we took ideas from it)
and also currently out of date.

See #740, #805, #871.

mvdan/sh,mvdan/sh,mvdan/sh,mvdan/sh