Add openssh package to build Docker image

This is work around for non-working HTTPS authorization in GitLab (not generally but for our specific case) allowing cloning repository using SSH. Problem can be with SSH keys, but it can be managed by using Docker volumes in config.toml (adding something like volumes = ["/cache", "/root/.ssh:/root/.ssh:ro"] in [runners.docker] section in config.toml).

The only change is adding ssh executable to the build image. I tried to build this image locally, but it's always rewritten to the original state by gitlab-ci-multi-runner pulling from Docker hub after each commit.