Why does SSH need a passphrase for a passphrase-less key?

Question

I have a SSH Key that uses no passphrase. I want to use that key to pull a private git repository when building a Docker container.

I successfully managed to get the key into the container at build time but now SSH fails because it can't open /dev/tty to ask for the key's passphrase. The key doesn't have one, as mentioned.

Here's some of the SSH output (-v)

debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey
debug1: Next authentication method: publickey
debug1: Offering RSA public key: /root/.ssh/id_rsa
debug1: Server accepts key: pkalg ssh-rsa blen 279
debug1: key_load_private_type: incorrect passphrase supplied to decrypt private key
debug1: read_passphrase: can't open /dev/tty: No such device or address
debug1: No more authentication methods to try.
Permission denied (publickey).
fatal: Could not read from remote repository.

Answer 1

Your key file might be corrupt in some way. ssh (at least some versions) will prompt for a passphrase any time it can't make sense of the key file:

$ dd if=/dev/urandom of=key bs=1500 count=1
1+0 records in
1+0 records out
1500 bytes transferred in 0.000190 secs (7893922 bytes/sec)
$ chmod 600 key
$ ssh -i key foo@localhost
Enter passphrase for key 'key':