x509: cannot verify signature: algorithm unimplemented

A GitLab instance is behind sha256 TLS. Gitlab Runner fails a SSL handshake with following error:

x509: cannot verify signature: algorithm unimplemented

Relevant details of certificate:

    Signature Algorithm: rsassaPss
         Hash Algorithm: sha256
         Mask Algorithm: mgf1 with sha256
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (2048 bit)

Might be related:

• http://stackoverflow.com/questions/25008571/golang-issue-x509-cannot-verify-signature-algorithm-unimplemented-on-net-http
• http://bridge.grumpy-troll.org/2014/05/golang-tls-comodo/
• https://www.imperialviolet.org/2014/05/14/sha256.html

Using: docker image gitlab/gitlab-runner:v1.6.1