ci(Dockerfile): add Go SDK to resulting image

Dockerfile

@@ -1,19 +1,16 @@
 ARG GO_VERSION=latest
 
-FROM golang:$GO_VERSION as builder
-
+FROM golang:$GO_VERSION AS builder
 WORKDIR /go/src/app
-
 COPY . .
-
 RUN go mod download
-
 RUN CGO_ENABLED=0 go build -o /go/bin/ogen ./cmd/ogen/main.go
 
 FROM scratch
-
+# We need go in resulting image to run goimports.
+COPY --from=builder /usr/local/go/ /usr/local/go/
+ENV PATH="/usr/local/go/bin:${PATH}"
+# Copy built binary.
 WORKDIR /
-
 COPY --from=builder /go/bin/ogen ./ogen
-
 ENTRYPOINT ["./ogen"]