Dockerfile: Use unversioned golang:alpine image

[cfergeau]

The Dockerfile hardcodes the use of golang:1.16-alpine.

Both golang 1.16 and the alpine version used in this image are unmaintained at this point, with most likely some known unfixed security issues.

This PR switches to an unversioned golang/alpine image, which should ensure we always use an up-to-date image. We can switch back to a versioned image if this causes issues and it turns out we need more predictability on the golang version used.