Remove file_md5 column from virtual_registries_container_cache_entries table

Summary

Remove the file_md5 column from the virtual_registries_container_cache_entries table following the three-release process for avoiding downtime in migrations.

Why remove file_md5?

The file_md5 column is not needed - we use file_sha1 for container images.

Three-Release Process

Following the guidelines in Avoiding downtime in migrations.

Release M (18.7) - Ignore the column

• Add ignore_column :file_md5 directive to the model
• Remove all code references to file_md5
• MR: Ignore file_md5 column in VirtualRegistries::Co... (!213182 - merged)

Release M+1 (18.8) - Drop the column

• Create migration to drop the file_md5 column from the database
• MR: Draft: Drop file_md5 column from VirtualRegistr... (!213999)

Release M+2 (18.9) - Remove ignore directive

• Remove the ignore_column directive from the model
• MR: TBD

Related

Related to !205956 (merged) (comment 2790898109)