I'm still having issues... I've been digging into it a bit more and I think the issue is with permissions on the server.

I've run tests against other repos (GitHub, a different GitLab repo) with the same key and it seems to be working fine. So I'm fairly sure the issue isn't with the key or with the way I'm testing validation (ssh -vvvt git@git.mdbootstrap.com)

It sounds (from the internet) like there could be a couple different things to look at:

• Can someone look at the authorized_keys file to see if my public key was added?
• Can someone look at the auth.log to see if there are errors from my attempts to connect? (most recent attempt was at 12:21pm MST on Mar 23, 2018)
• https://forum.gitlab.com/t/authorizing-with-ssh-key-against-gitlab/11411/2
• https://superuser.com/questions/1137438/ssh-key-authentication-fails

Here's a dump of the test connect attempt:

debug3: load_hostkeys: loaded 1 keys from 37.187.244.81
debug1: Host 'git.mdbootstrap.com' is known and matches the ECDSA host key.
debug1: Found key in /c/Users/USERNAME/.ssh/known_hosts:1
debug3: send packet: type 21
debug2: set_newkeys: mode 1
debug1: rekey after 4294967296 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug3: receive packet: type 21
debug1: SSH2_MSG_NEWKEYS received
debug2: set_newkeys: mode 0
debug1: rekey after 4294967296 blocks
debug2: key: /c/Users/USERNAME/.ssh/id_rsa_mdbootstrap (0x600071e30), agent                <============= key is properly registered with ssh-agent
debug2: key: /c/Users/USERNAME/.ssh/id_rsa (0x0)
debug2: key: /c/Users/USERNAME/.ssh/id_dsa (0x0)
debug2: key: /c/Users/USERNAME/.ssh/id_ecdsa (0x0)
debug2: key: /c/Users/USERNAME/.ssh/id_ed25519 (0x0)
debug3: send packet: type 5
debug3: receive packet: type 6
debug2: service_accept: ssh-userauth
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug3: send packet: type 50
debug3: receive packet: type 51
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
debug3: start over, passed a different list publickey,gssapi-keyex,gssapi-with-mic,password
debug3: preferred publickey,keyboard-interactive,password
debug3: authmethod_lookup publickey
debug3: remaining preferred: keyboard-interactive,password
debug3: authmethod_is_enabled publickey
debug1: Next authentication method: publickey
debug1: Offering public key: RSA SHA256:eM7vEpPlovnBxiycTk+lrmr5XprG+T0rZItmtWJ8CYA /c/Users/USERNAME/.ssh/id_rsa_mdbootstrap                         <========= sending public key
debug3: send_pubkey_test
debug3: send packet: type 50
debug2: we sent a publickey packet, wait for reply
debug3: receive packet: type 51
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
debug1: Trying private key: /c/Users/USERNAME/.ssh/id_rsa
debug3: no such identity: /c/Users/USERNAME/.ssh/id_rsa: No such file or directory
debug1: Trying private key: /c/Users/USERNAME/.ssh/id_dsa
debug3: no such identity: /c/Users/USERNAME/.ssh/id_dsa: No such file or directory
debug1: Trying private key: /c/Users/USERNAME/.ssh/id_ecdsa
debug3: no such identity: /c/Users/USERNAME/.ssh/id_ecdsa: No such file or directory
debug1: Trying private key: /c/Users/USERNAME/.ssh/id_ed25519
debug3: no such identity: /c/Users/USERNAME/.ssh/id_ed25519: No such file or directory
debug2: we did not send a packet, disable method                                                                                  <====================================== Something went wrong
debug3: authmethod_lookup password
debug3: remaining preferred: ,password
debug3: authmethod_is_enabled password
debug1: Next authentication method: password
git@git.mdbootstrap.com's password: