Medium: docker: Correction of the mistake of the variable name.

I know that a correction of docker RA still remains.

• https://github.com/ClusterLabs/resource-agents/pull/649
• https://github.com/ClusterLabs/resource-agents/pull/651

However, handling of the variable of next commit is like the mistake on
latest docker RA.

https://github.com/ClusterLabs/resource-agents/commit/5a6a77f963d90d818ca556104790a7e7fc9d84fa

But I do not know a lot about docker RA, this correction may be wrong.

Best Regards,
Hideo Yamauchi.