root@5a5ed5d6a9d9:/# apt-cache policy librdkafka1
librdkafka1:
  Installed: 0.9.3-1
  Candidate: 1.3.0~1confluent5.4.0-1
  Version table:
     1.3.0~1confluent5.4.0-1 500
        500 https://packages.confluent.io/deb/5.4 stable/main amd64 Packages
 *** 0.9.3-1 500
        500 http://deb.debian.org/debian stretch/main amd64 Packages
        100 /var/lib/dpkg/status

root@5a5ed5d6a9d9:/# apt install librdkafka1
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 librdkafka1 : Depends: libssl1.0.0 (>= 1.0.0) but it is not installable
E: Unable to correct problems, you have held broken packages.