Stop building Docker images for Debian 8

Debian archive signing key for Jessie has expired, and building this
Docker container now results in an error. Old Docker images are good
enough for our tests, it's not like Jessie packages are going to get
updated ever again.

Author: sio

.github/workflows/docker.yml

@@ -21,7 +21,6 @@ jobs:
         tag:
           - bash-3.2
           - centos-7
-          - debian-8
           - debian-9
           - debian-10
           - debian-11