When running the alpine docker container the first time and attempting an openssh install, the following happens:
ole@T:~$ docker run -it --rm alpine /bin/ash
/ # apk add openssh
WARNING: Ignoring APKINDEX.d3812b7e.tar.gz: No such file or directory
WARNING: Ignoring APKINDEX.bb2c5760.tar.gz: No such file or directory
ERROR: unsatisfiable constraints:
openssh (missing):
required by: world[openssh]
How should openssh be installed?
Run apk update first. The below paste contains 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
As the answer is already given but I will add up something.
In the new version, If you want to install something without caching things locally, which is recommended for keeping your containers small, include the --no-cache flag. Example:
apk add --no-cache openssh
This is a small gain, it keeps you from having the common rm -rf /var/cache/apk/* at the end of your Dockerfile.
Dockerfile for your reference
FROM alpine:3
RUN apk add --update --no-cache openssh
CMD ["ssh"]