chore: add build target to Makefile & builder to Dockerfile

Dockerfile

@@ -1,10 +1,29 @@
-FROM alpine:3.15.0
+# Build the binary
+FROM docker.io/library/golang:1.18 as builder
+
+WORKDIR /workspace
+# Copy the Go Modules manifests
+COPY go.mod go.mod
+COPY go.sum go.sum
+# cache deps before building and copying source so that we don't need to re-download as much
+# and so that source changes don't invalidate our downloaded layer
+RUN go mod download
+
+# Copy the go source
+COPY main.go main.go
+COPY cmd/ cmd/
+COPY phpfpm/ phpfpm/
+
+# Build
+RUN CGO_ENABLED=0 go build -a -o php-fpm_exporter main.go
+
+FROM docker.io/library/alpine:3.15.0
 
 ARG BUILD_DATE
 ARG VCS_REF
 ARG VERSION
 
-COPY php-fpm_exporter /
+COPY --from=builder /workspace/php-fpm_exporter .
 
 EXPOSE     9253
 ENTRYPOINT [ "/php-fpm_exporter", "server" ]

Makefile

@@ -7,6 +7,9 @@ help: ## List targets & descriptions
 deps: ## Get dependencies
 	go get -d -v ./...
 
+build: deps ## Build the binary
+	go build -a -o php-fpm_exporter main.go
+
 test: ## Run tests
 	go test -short ./...
 
@@ -26,4 +29,4 @@ lint: ## Run linters
 	golangci-lint run
 
 fmt: ## Fix formatting issues
-	goimports -w .
+	goimports -w .