"Host key verification failed" when building from private git repo

[aidan-mundy]

When building a docker image using BuildImage and a BuildImageOptions containing a Remote value (instead of ContextDir), any git repo accessed over the git/ssh protocol returns API error (500): error fetching: Host key verification failed. Git repo access over the https protocol works fine for public repos, but private repos error with API error (500): error fetching: fatal: could not read Username for 'https://HOSTNAMEHERE': No such device or address

Related to #376
Related to docker/compose#3038
Related to docker/compose#2856

[stale]

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