Dockerfile: update ALPINE_VERSION to 3.17

Official Golang images are now only available for 3.18 and 3.17;
3.18 doesn't look to play well with gotestsum, so sticking to
an older version.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit acb248f8d5)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

dockerfiles/Dockerfile.authors

@@ -1,6 +1,6 @@
 # syntax=docker/dockerfile:1
 
-ARG ALPINE_VERSION=3.16
+ARG ALPINE_VERSION=3.17
 
 FROM alpine:${ALPINE_VERSION} AS gen
 RUN apk add --no-cache bash git