Tests failing after official Docker Distribution registry (registry:2) removed htpasswd

[chanseokoh]

Our integration tests run

docker run --rm --entrypoint htpasswd registry:2 -Bbn username password

On my desktop where it cached an old version of registry:2, the command works.

$ docker run --rm --entrypoint htpasswd registry:2 -Bbn username password
username:$2y$05$RO8TSDjphyaccxuztK6L.uQrJ0csk.YpAhfvS7cm2d5Z/cI3PAZm6

$ docker images
REPOSITORY                               TAG                 IMAGE ID            CREATED             SIZE
registry                                 2                   708bc6af7e5e        4 months ago        25.8MB

However, the new registry:2 image they created 27 hours ago fails, as it doesn't have htpasswd.

$ docker run --rm --entrypoint htpasswd registry:2 -Bbn username password
docker: Error response from daemon: OCI runtime create failed: container_linux.go:346: starting container process caused "exec: \"htpasswd\": executable file not found in $PATH": unknown.
$ docker images
REPOSITORY               TAG                  IMAGE ID            CREATED             SIZE
registry                 2                    2d4f4b5309b1        27 hours ago        26.2MB

Here's a sample Kokoro build failure log.

[chanseokoh]

If we use it only to run htpasswd, then maybe we can just switch to any image providing htpasswd.

Alternatively, we could pin to an older registry:2 image (only if the image is for having htpasswd).

[chanseokoh]

Fixed by #2538.