The version of libssh2 within CentOS 7 lacks support for the more-recent versions of SSH key exchange methods and MACs. As the guacamole/guacd Docker image uses CentOS 7, it also lacks this support.
Currently, the following key exchange methods are supported:
and the following MACs are supported:
More recent libssh2 includes support for SHA2 and curve25519. See https://wiki.mozilla.org/Security/Guidelines/OpenSSH for more information and references.