Add container env var with value docker for AmazonLinux

Without this, we get an error of the form "Failed to get D-Bus
connection: No such file or directory" when Ansible attempts to run
any systemctl commands.

I'm not sure exactly why this is necessary, but I came across it here:
moby/moby#7459

molecule/default/molecule-with-systemd.yml

@@ -17,6 +17,8 @@ lint: |
 platforms:
   - cgroupns_mode: host
     command: /lib/systemd/systemd
+    env:
+      container: docker
     image: geerlingguy/docker-amazonlinux2-ansible:latest
     name: amazonlinux2-systemd
     platform: amd64