install openssh-client package for ssh command-line utilities

[dmitris]

What are you trying to accomplish?

Install openssh-client package as the initial step of the changes to propagate SSH credentials into the development / updater container and use them if necessary for repository access. Related to issue #11544.

Anything you want to highlight for special attention from reviewers?

This is the initial step to add the feature. For now, the apt package openssl-client is installed but not yet used. See the issue for the proposed steps to follow this one.

How will you know you've accomplished your goal?

After starting the development shell for example with bin/docker-dev-shell go_modules, one can run:

$ which ssh-add
/usr/bin/ssh-add

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.