fix(docker): install buf in Dockerfile for proto generation

Add buf installation to fix docker build error.
The Makefile now uses 'buf generate' so buf must be available
in the build environment.

Author: whywaita

Dockerfile

@@ -4,6 +4,7 @@ WORKDIR /go/src/github.com/whywaita/myshoes
 
 RUN go install google.golang.org/protobuf/cmd/protoc-gen-go@v1.34.2
 RUN go install google.golang.org/grpc/cmd/protoc-gen-go-grpc@v1.5.1
+RUN go install github.com/bufbuild/buf/cmd/buf@latest
 RUN apt-get update -y \
     && apt-get install -y protobuf-compiler