Required by: world[php7-pecl-ssh2]

Hello I am new to Docker. I am trying to install ssh2 library.
Ran the following commands without any problem:
docker exec -u root -t -i $CONTAINER apk update
docker exec -u root -t -i $CONTAINER apk add libssh2-dev autoconf build-base

Docker ver 2.1
OS . - MAC OS High Sierra

Tried doing the following:
docker exec -u root -t -i 9041fe410960 apk add php7-pecl-ssh2
Got the following:
ERROR: unsatisfiable constraints:
php7-pecl-ssh2 (missing):
required by: world[php7-pecl-ssh2]

I understand I need to have access to some Global Respository. Many posts pointed to making changes in /etc/apk/repository folder. I dont see any such folder in my Docker folder under Applications.

Please help!