Update to Go 1.15.2 toolchain.

- Use a "tool dependency" to specify the protoc-gen-go, protoc-gen-grpc-gateway, and protoc-gen-openapiv2 required versions. See [here](https://github.com/golang/go/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module).

### Changed

- Build with Go 1.15.2 release.

- Update to Protobuf 1.4.2, GRPC 1.32.0, and GRPC-Gateway 2.0.0-beta.5. This enables us to take advantage of the new Protobuf runtime. See [here](https://blog.golang.org/protobuf-apiv2).

- Replace shell script with Go generate commands to run protoc toolchain.

- Update protocol definitions to remove warnings from stricter Go package import paths. See [here](https://developers.google.com/protocol-buffers/docs/reference/go-generated#package).

# docker build "$PWD" --build-arg commit="$(git rev-parse --short HEAD)" --build-arg version=v2.1.1 -t heroiclabs/nakama:2.1.1

# docker build "$PWD" --build-arg commit="$(git rev-parse --short HEAD)" --build-arg version="v2.1.1-$(git rev-parse --short HEAD)" -t heroiclabs/nakama-prerelease:"2.1.1-$(git rev-parse --short HEAD)"

FROM golang:1.15.0-buster as builder

FROM golang:1.15.2-buster as builder

ARG commit

ARG version

# docker build "$PWD" --file ./Dockerfile.pluginbuilder --build-arg commit="$(git rev-parse --short HEAD)" --build-arg version=v2.1.1 -t heroiclabs/nakama-pluginbuilder:2.1.1

# docker build "$PWD" --file ./Dockerfile.pluginbuilder --build-arg commit="$(git rev-parse --short HEAD)" --build-arg version="v2.1.1-$(git rev-parse --short HEAD)" -t heroiclabs/nakama-prerelease:"2.1.1-$(git rev-parse --short HEAD)"

FROM golang:1.15.0-buster as builder

FROM golang:1.15.2-buster as builder

MAINTAINER Heroic Labs <support@heroiclabs.com>