reorganization of _examples and add some new examples such as iris+groupcache+mysql+docker

Former-commit-id: ed635ee95de7160cde11eaabc0c1dcb0e460a620

_examples/kafka-api/Dockerfile

@@ -0,0 +1,17 @@
+# docker build -t myapp . 
+# docker run --rm -it -p 8080:8080 myapp:latest
+FROM golang:latest AS builder
+RUN apt-get update
+ENV GO111MODULE=on \
+    CGO_ENABLED=0 \
+    GOOS=linux \
+    GOARCH=amd64
+WORKDIR /go/src/app
+COPY go.mod .
+RUN go mod download
+COPY . .
+RUN go install
+
+FROM scratch
+COPY --from=builder /go/bin/myapp .
+ENTRYPOINT ["./myapp"]