add openssh-client to devcontainer

[hacki11]

PR Checklist:

• [X] Provide a meaningful description to this PR or mention which issues this fixes.
• [X] Ensure the project builds with npm run build
• [ ] Add tests for your change. This includes negative tests (i.e. inputs that need to fail) as well as baseline tests (i.e. how should the directory structure look like?).
• [X] Run the test suite with npm test
• [ ] N/A If there are baseline changes, review them and make a separate commit for them with the comment "accept baselines" if they are desired changes
• [ ] N/A If you added a required option, also add it to the template creation (.github/create_templates.ts)
• [X] Add a detailed migration description to docs/updates explaining what the user needs to do when manually updating an existing project
• [x] Add your changes to CHANGELOG.md (referencing the migration description and this PR or the issue you fixed)

Description:
As a developer, I want to commit & push to the adapters git repository I cloned with a ssh url instead of https. The current iobroker docker image does not include openssh-client, so ssh is not working. This PR changes that.

[hacki11]

How would you test such a change? Is there a test which validates that the DevContainer is actually working?

[hacki11]

@AlCalzone anything open from your side or can we merge this?

[Apollon77]

@hacki11 I needed to merge something in the docker part. Please check again before I merge!