make: fix docker warnings

These fixes stop Docker from warning about the deprecated MAINTAINER
instruction and legacy key/value format.

Author: jtobin

make/builder.Dockerfile

@@ -1,10 +1,10 @@
 FROM golang:1.24.9-bookworm@sha256:737b40b61ce956d738bed59f18ba854d8d67e7a4c4fa63f64437f4c70247ac5b
 
-MAINTAINER Olaoluwa Osuntokun <laolu@lightning.engineering>
+LABEL maintainer="Olaoluwa Osuntokun <laolu@lightning.engineering>"
 
 # Golang build related environment variables that are static and used for all
 # architectures/OSes.
-ENV GODEBUG netdns=cgo
+ENV GODEBUG=netdns=cgo
 ENV CGO_ENABLED=0
 
 # Set up cache directories. Those will be mounted from the host system to speed