Use dockertest v3 consistently (#167)

go.mod

@@ -45,7 +45,6 @@ require (
 	github.com/mitchellh/mapstructure v1.3.2
 	github.com/niemeyer/pretty v0.0.0-20200227124842-a10e7caefd8e // indirect
 	github.com/oligot/go-mod-upgrade v0.1.2
-	github.com/ory/dockertest v3.3.5+incompatible
 	github.com/ory/dockertest/v3 v3.6.0
 	github.com/pelletier/go-toml v1.7.0 // indirect
 	github.com/pires/go-proxyproto v0.1.3

internal/db/db.go

@@ -13,7 +13,7 @@ import (
 	"github.com/hashicorp/watchtower/internal/db/migrations"
 	"github.com/jinzhu/gorm"
 	"github.com/lib/pq"
-	"github.com/ory/dockertest"
+	"github.com/ory/dockertest/v3"
 )
 
 type DbType int