Another attempt

Dockerfile

@@ -5,7 +5,8 @@ FROM alpine:latest
 # - Fix default group (1000 does not exist)
 # - OpenSSH needs /var/run/sshd to run
 # - Remove generic host keys, entrypoint generates unique keys
-RUN apk add --no-cache bash shadow@community openssh openssh-sftp-server && \
+RUN echo "@community http://dl-cdn.alpinelinux.org/alpine/edge/community" >> /etc/apk/repositories && \
+    apk add --no-cache bash shadow@community openssh openssh-sftp-server && \
     sed -i 's/GROUP=1000/GROUP=100/' /etc/default/useradd && \
     mkdir -p /var/run/sshd && \
     rm -f /etc/ssh/ssh_host_*key*