Add support for ssh forwarding to docker

[dilpreet92]

• this will enable clone private gem repositories or in general forwarding ssh connection to docker

[dhh]

What benefit do you see here over using BUNDLE_GITHUB__COM with a Docker secret, as described in the README? https://github.com/mrsked/mrsk/#using-build-secrets-for-new-images

[dilpreet92]

What benefit do you see here over using BUNDLE_GITHUB__COM with a Docker secret, as described in the README? https://github.com/mrsked/mrsk/#using-build-secrets-for-new-images

there might be some cases where private gems are being accessed using ssh and not https:.
gem 'abc', git: 'git@github.com:dilpreet92/xxx.git'
In order for SSH to work with Bundler, the user executing bundle install needs to have an authorized private key for the Github repo, so a common pattern is to use a machine user on CI, that has read access to all the repos it needs access to.

[djmb]

Support for this was added in #434