how to use remote Docker host

@deansheather

Create a new remote docker template similar to docker-image-builds

Remote docker host should be specified as ssh://user@host
Key to access the host should be already available on the deployment in ~/.ssh/KEY
For services like fly.io, railway.app, render.com, where putting the key inside the Dockerfile is not a good solution. The private key can be created as a secret or secured environment variable.
We can also create a managed sensitive terraform variable to store the key, which is not secure as values are stored unencrypted in the database. Thanks, @deansheather
Check for the value of the Secret/Environment variable and create ~/.ssh/KEY file with the correct permissions on the Coder host to be able to access the remote docker host using terraform local-exec

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