Update mysql

Changes:

- https://github.com/docker-library/mysql/commit/c273193: Update 5.6 to 5.6.51, debian 5.6.51-1debian9

library/mysql

@@ -14,7 +14,7 @@ GitCommit: 3fde29717aea785d4e37a97c9990ff0fda37e544
 Directory: 5.7
 File: Dockerfile.debian
 
-Tags: 5.6.50, 5.6
-GitCommit: b0a0e546712534b30ac13e8b2829f23654facbc5
+Tags: 5.6.51, 5.6
+GitCommit: c273193c14bb45daea2d27bb6ec51aa0b2b2ab9b
 Directory: 5.6
 File: Dockerfile.debian