Dockerfile.aarch64hub: install shadow package (#5481)

the shadow package contains the usermod(8) utility which is used by
start.sh and finalize.sh to configure the git user

Dockerfile.aarch64hub

@@ -19,7 +19,7 @@ RUN [ "cross-build-start" ]
 # Install system utils & Gogs runtime dependencies
 ADD https://github.com/tianon/gosu/releases/download/1.10/gosu-arm64 /usr/sbin/gosu
 RUN chmod +x /usr/sbin/gosu \
- && apk --no-cache --no-progress add ca-certificates bash git linux-pam s6 curl openssh socat tzdata go=1.10.1-r0
+ && apk --no-cache --no-progress add ca-certificates bash git linux-pam s6 curl openssh shadow socat tzdata go=1.10.1-r0