How to use SSH keys for private repo in Docker build

We have several private packages that are utilized by our package.json file within our Node application. We added our SSH key to Bamboo Shared Credentials in hopes that it will be accessible within the Docker container used to build the application. However, I continue to receive the following error when trying to build:

npm ERR! Host key verification failed.
npm ERR! fatal: Could not read from remote repository.

It doesn’t look like the SSH key is made available. I assume others have encountered this issue - how do you access private repos without storing the SSH keys as part of the Docker image?