Fix docker URIs with registry ports

[devinhuang2001]

Summary

• normalize the docker URI scheme before parsing the image reference
• cover docker URIs whose registry includes a port

Fixes #2536

Testing

• go test ./pkg/buildpack -run TestGetLocatorType -count=1 (blocked locally because the network cannot download Go modules from proxy.golang.org; setup failed before test execution)
• gofmt -d pkg/buildpack/locator_type.go pkg/buildpack/locator_type_test.go
• git diff --check