build: disambiguate base image names

to build with podman / buildah.

Author: pini-gh

Dockerfile

@@ -1,6 +1,6 @@
-FROM nginxproxy/docker-gen:0.14.0 AS docker-gen
+FROM docker.io/nginxproxy/docker-gen:0.14.0 AS docker-gen
 
-FROM alpine:3.20.0
+FROM docker.io/library/alpine:3.20.0
 
 ARG GIT_DESCRIBE="unknown"
 ARG ACMESH_VERSION=3.0.7