Cannot SSH using private key while building Docker image

Question

I'm having trouble checking out a private git repository hosted on GitHub during a Docker image build. The error from SSH in verbose mode is:

OpenSSH_6.6.1, OpenSSL 1.0.1f 6 Jan 2014
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: Applying options for *
debug1: Connecting to github.com [192.30.252.130] port 22.
debug1: Connection established.
debug1: permanently_set_uid: 0/0
debug1: identity file /root/.ssh/id_rsa type -1
debug1: identity file /root/.ssh/id_rsa-cert type -1
debug1: identity file /root/.ssh/id_dsa type -1
debug1: identity file /root/.ssh/id_dsa-cert type -1
debug1: identity file /root/.ssh/id_ecdsa type -1
debug1: identity file /root/.ssh/id_ecdsa-cert type -1
debug1: identity file /root/.ssh/id_ed25519 type -1
debug1: identity file /root/.ssh/id_ed25519-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.6.1p1 Ubuntu-2ubuntu2
debug1: Remote protocol version 2.0, remote software version libssh-0.6.0
debug1: no match: libssh-0.6.0
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-sha1 none
debug1: kex: client->server aes128-ctr hmac-sha1 none
debug1: sending SSH2_MSG_KEX_ECDH_INIT
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: RSA 16:27:ac:a5:76:28:2d:36:63:1b:56:4d:eb:df:a6:48
debug1: read_passphrase: can't open /dev/tty: No such device or address
Host key verification failed.

The /dev/tty device does seem to be present:

total 4
drwxr-xr-x   4 root root     340 Jul 22 17:12 .
drwxr-xr-x 130 root root    4096 Jul 22 17:12 ..
lrwxrwxrwx   1 root root      13 Jul 22 17:12 fd -> /proc/self/fd
crw-rw-rw-   1 root root  1,   7 Jul 22 17:12 full
c---------   1 root root 10, 229 Jul 22 17:12 fuse
lrwxrwxrwx   1 root root      11 Jul 22 17:12 kcore -> /proc/kcore
crw-rw-rw-   1 root root  1,   3 Jul 22 17:12 null
lrwxrwxrwx   1 root root       8 Jul 22 17:12 ptmx -> pts/ptmx
drwxr-xr-x   2 root root       0 Jul 22 17:12 pts
crw-rw-rw-   1 root root  1,   8 Jul 22 17:12 random
drwxrwxrwt   2 root root      40 Jul 22 17:12 shm
lrwxrwxrwx   1 root root      15 Jul 22 17:12 stderr -> /proc/self/fd/2
lrwxrwxrwx   1 root root      15 Jul 22 17:12 stdin -> /proc/self/fd/0
lrwxrwxrwx   1 root root      15 Jul 22 17:12 stdout -> /proc/self/fd/1
crw-rw-rw-   1 root root  5,   0 Jul 22 17:12 tty
crw-rw-rw-   1 root root  1,   9 Jul 22 17:12 urandom
crw-rw-rw-   1 root root  1,   5 Jul 22 17:12 zero

Here is a stripped down version of my Dockerfile illustrating my test:

FROM ubuntu:trusty

ADD . /my_app
ADD ./config/ssh/docker_ssh_key /root/.ssh/id_rsa

RUN ls -al /dev
RUN ssh -t -t -v [email protected]

CMD bundle exec thin -p $PORT -R config.ru start

I have tested the build and know that the key does work. I can access the git repository just fine if I build the image without the RUN commands and start a container with an interactive shell.

I have found some other questions with similar errors messages. But they are for missing /dev/tty which doesn't seem to be the case here. Any ideas?

Answer 1

You need to accept the Github host key. Use ssh-keyscan:

ssh-keyscan -t rsa github.com 2>&1 >> /root/.ssh/known_hosts