Use official golang docker image in CI

[sagikazarmark]

Currently we use mysqldiff to check the consistency between migration scripts and gorm automigration.

Unfortunately, Ubuntu Bionic and Debian Stretch were the last distributions with a package containing mysqldiff.

Since Go 1.13 images use newer distributions, we temporarily had to switch to a custom image based on bionic.

For the long term, we need to find another way to run those tests and get rid of the custom image.