guacd docker container can't validate RDP certificate

guacd[5]: INFO: Guacamole proxy daemon (guacd) version 1.0.0 started
guacd[5]: INFO: Listening on host 0.0.0.0, port 4822
guacd[5]: INFO: Creating new client for protocol "rdp"
guacd[5]: INFO: Connection ID is "$8791f12e-0d99-4aac-8ddf-b893c60e387c"
guacd[7]: INFO: Security mode: ANY
guacd[7]: INFO: Resize method: display-update
guacd[7]: INFO: User "@4dae41b2-c439-4175-9543-39509c737706" joined connection "$8791f12e-0d99-4aac-8ddf-b893c60e387c" (1 users now present)
guacd[7]: INFO: Loading keymap "base"
guacd[7]: INFO: Loading keymap "en-us-qwerty"
connected to winpc.[domainname].com:3389
creating directory /root/.config/freerdp
creating directory /root/.config/freerdp/certs
creating directory /root/.config/freerdp/server
certificate_store_open: error opening [/root/.config/freerdp/known_hosts] for writing
guacd[7]: INFO: Certificate validation failed
tls_connect: certificate not trusted, aborting.
Error: protocol security negotiation or connection failure
guacd[7]: ERROR:        Error connecting to RDP server
guacd[7]: INFO: User "@4dae41b2-c439-4175-9543-39509c737706" disconnected (0 users remain)
guacd[7]: INFO: Last user of connection "$8791f12e-0d99-4aac-8ddf-b893c60e387c" disconnected

# openssl s_client -showcerts -connect winpc.[domainname].com:3389
CONNECTED(00000003)
depth=2 C = GB, ST = Greater Manchester, L = Salford, O = COMODO CA Limited, CN = COMODO RSA Certification Authority
verify return:1
depth=1 C = GB, ST = Greater Manchester, L = Salford, O = COMODO CA Limited, CN = COMODO RSA Domain Validation Secure Server CA
verify return:1
depth=0 OU = Domain Control Validated, OU = PositiveSSL, CN = winpc.[domainname].com
verify return:1
---
Certificate chain
 0 s:/OU=Domain Control Validated/OU=PositiveSSL/CN=winpc.[domainname].com
   i:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Domain Validation Secure Server CA
-----BEGIN CERTIFICATE-----
[Cert Data]
-----END CERTIFICATE-----
 1 s:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Domain Validation Secure Server CA
   i:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Certification Authority
-----BEGIN CERTIFICATE-----
[Cert Data]
-----END CERTIFICATE-----
---
Server certificate
subject=/OU=Domain Control Validated/OU=PositiveSSL/CN=winpc.[domainname].com
issuer=/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Domain Validation Secure Server CA
---
No client certificate CA names sent
Peer signing digest: SHA256
Server Temp Key: ECDH, P-384, 384 bits
---
SSL handshake has read 4333 bytes and written 447 bytes
---
New, TLSv1/SSLv3, Cipher is ECDHE-RSA-AES256-GCM-SHA384
Server public key is 4096 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : ECDHE-RSA-AES256-GCM-SHA384
    Session-ID: 01310000F93A78635295B0F5A5458E9AEC16BF70B72E28052D201B6B8DE6661B
    Session-ID-ctx:
    Master-Key: FFFDC45C96C282A330BF878272FD243783425508ED6CB43492C127431492B04089AC8630E509B42DD909DF042286F913
    Key-Arg   : None
    Krb5 Principal: None
    PSK identity: None
    PSK identity hint: None
    Start Time: 1547126917
    Timeout   : 300 (sec)
    Verify return code: 0 (ok)
---

root@a218bfbd187e:/# dpkg -l | grep cert
root@a218bfbd187e:/#
root@a218bfbd187e:/# ls /etc/ssl/certs/
ls: cannot access '/etc/ssl/certs/': No such file or directory