In case this helps anyone, I realized my issue.
My DataSource was set to "username:password@127.0.0.1:3306/mattermost" but it should have been "username:password@tcp(127.0.0.1:3306)/mattermost".
In case this helps anyone, I realized my issue.
My DataSource was set to "username:password@127.0.0.1:3306/mattermost" but it should have been "username:password@tcp(127.0.0.1:3306)/mattermost".