`ch-image build`: pass through SSH agent

SSH_AUTH_SOCK is not currently passed into the build by default, so one must add ARG SSH_AUTH_SOCK to the Dockerfile and --build-arg=SSH_AUTH_SOCK to the build command line in order for SSH authentication via the agent to work.

We should instead pass through this variable by default, like the proxy variables.

I thought we already did this, but I guess not, or maybe something changed.

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