The template used was broken with Go 1.14, which has some stricter
rules, introduced in https://golang.org/cl/206124

```
root@b5beaed18589:/go/src/github.com/docker/docker# ./hack/generate-swagger-api.sh
2020/02/07 15:12:01 trying to read config from /go/src/github.com/docker/docker/api/swagger-gen.yaml
2020/02/07 15:12:02 rendering 1 templates for model ErrorResponse
2020/02/07 15:12:02 name field ErrorResponse
2020/02/07 15:12:02 package field types
2020/02/07 15:12:02 creating "error_response.go" in "api/types" as definition
model: template: schematype:2:18: executing "schemaType" at <(len .AllOf) gt 0>: can't give argument to non-function len .AllOf
```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>