chore(deps): update lsiobase/alpine docker tag to v3.15

Dockerfile.port

-FROM lsiobase/alpine:3.14
+FROM lsiobase/alpine:3.15
 
 LABEL maintainer="Griefed <griefed@griefed.de>"
 LABEL description="Dockerfile to build an image of D-Zone which is to be accessed with IP:PORT"

Dockerfile.proxy

-FROM lsiobase/alpine:3.14
+FROM lsiobase/alpine:3.15
 
 LABEL maintainer="Griefed <griefed@griefed.de>"
 LABEL description="Dockerfile to build an image of D-Zone which is to be accessed with a reverse proxy like NGINX"