Start a Zabbix agent container as follows:
docker run --name some-zabbix-agent -e ZBX_HOSTNAME="some-hostname" -e ZBX_SERVER_HOST="some-zabbix-server" -d zabbix/zabbix-agent:tag
Where some-zabbix-agent is the name you want to assign to your container, some-hostname is the hostname, it is Hostname parameter in Zabbix agent configuration file, some-zabbix-server is IP or DNS name of Zabbix server or proxy and tag is the tag specifying the version you want. See the list above for relevant tags, or look at the full list of tags.
This image now supports setting the desired speedtest server via docker environment variable SPEEDTEST_SERVER. This should make it substantially more useful to people around the world.