minor hadolint fixes by pinning versions and use copy not add

Author: halkeye

Dockerfile

@@ -2,11 +2,11 @@ FROM debian:bullseye
 
 RUN apt-get update \
 	&& DEBIAN_FRONTEND=noninteractive \
-	   apt-get install -y --no-install-recommends pdns-server pdns-backend-pgsql \
+	   apt-get install -y --no-install-recommends pdns-server=4.3.1-2 pdns-backend-pgsql=4.3.1-2  \
 	&& rm -rf /etc/powerdns/pdns.d/* /var/lib/apt/lists/*
 
-ADD pdns.conf /etc/powerdns/
-ADD init /sbin/
+COPY pdns.conf /etc/powerdns/
+COPY init /sbin/
 
 EXPOSE 53/tcp 53/udp