How to install OpenSSH on Alpine?

Run apk update first.

A complete example:

    ole@T:~$ docker run -it --rm alpine /bin/ash
    / # apk update
    fetch http://dl-4.alpinelinux.org/alpine/v3.3/main/x86_64/APKINDEX.tar.gz
    fetch http://dl-4.alpinelinux.org/alpine/v3.3/community/x86_64/APKINDEX.tar.gz
    v3.3.1-97-g109077d [http://dl-4.alpinelinux.org/alpine/v3.3/main]
    v3.3.1-59-g48b0368 [http://dl-4.alpinelinux.org/alpine/v3.3/community]
    OK: 5853 distinct packages available
    / # apk add openssh
    (1/3) Installing openssh-client (7.1_p2-r0)
    (2/3) Installing openssh-sftp-server (7.1_p2-r0)
    (3/3) Installing openssh (7.1_p2-r0)
    Executing busybox-1.24.1-r7.trigger
    OK: 8 MiB in 14 packages