Make SSH keys available in Docker Container

fixes #303 fixes #326

[x] Is the title of the Pull Request correct?
[x] Is the title of the corresponding issue correct?
[x] Have you updated the changelog?
[x] Have you checked to ensure there aren't other open Pull Requests for the same update/change? There are other pull requests currently
[x] Are you mentioning the issue which this PullRequest fixes ("Fixes...")
[x] Before you merge don't forget to run all tests for all Exasol version, by adding [run all tests] to the commit message
[x] Are the CLI usage examples up to date?

exasol / integration-test-docker-environment