Update test using runner docker and ssh client

What does this MR do?

gitlab-runner!1781 (merged) removed openssh-client from the gitlab/gitlab-runner:ubuntu docker image. That caused an e2e test to fail because the test relies on ssh-keyscan, which was in the docker image until that MR.

This MR switches to the alpine image (because it's smaller), and installs openssh-client as part of the test script.

Closes #223622 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team