build: use buildkit cache for faster docker builds

Dockerfile

@@ -1,18 +1,25 @@
 # syntax=docker/dockerfile:1.0-experimental
 FROM golang:1.15-alpine as builder
 ARG VERSION
+ENV GOCACHE "/go-build-cache"
 WORKDIR /src
 
+# Why: Pinning versions in a docker container isn't
+# really worth the effort.
+# hadolint ignore=DL3018
 RUN apk add --no-cache make bash git libc-dev gcc
 
-# Add go.mod and go.sum first to maximize caching
-COPY ./go.mod ./go.sum ./
-RUN go mod download
-
+# Copy all files into our docker container.
 COPY . .
 
+# --mount here allows us to cache the packages even if
+# it's invalidated by go.mod,go.sum
+RUN --mount=type=cache,target=/go/pkg make dep
+
 # Build our application
-RUN make build APP_VERSION=${VERSION}
+# --mount here allows us to save go build cache across builds
+# but also needed to use the package cache above
+RUN --mount=type=cache,target=/go-build-cache --mount=type=cache,target=/go/pkg make build APP_VERSION=${VERSION}
 
 FROM alpine:3.12
 ENTRYPOINT ["/usr/bin/localizer"]

Makefile

@@ -3,7 +3,7 @@ GO             ?= go
 SHELL          := /usr/bin/env bash
 GOOS           := $(shell go env GOOS)
 GOARCH         := $(shell go env GOARCH)
-PKG            := $(GO) mod download
+PKG            := $(GO) mod download -x
 # TODO(jaredallard): infer from Git tag
 APP_VERSION    := $(shell git describe --match 'v[0-9]*' --tags --abbrev=0 --always HEAD)
 LDFLAGS        := -w -s
@@ -12,7 +12,6 @@ GOPROXY        := https://proxy.golang.org
 GO_EXTRA_FLAGS := -v
 TAGS           :=
 BINDIR         := $(CURDIR)/bin
-BIN_NAME       := bootstraper
 PKGDIR         := github.com/jaredallard/localizer
 CGO_ENABLED    := 1
 BENCH_FLAGS    := "-bench=Bench $(BENCH_FLAGS)"
@@ -61,9 +60,9 @@ gobuild:
 	mkdir -p $(BINDIR)
 	GOPROXY=$(GOPROXY) CGO_ENABLED=$(CGO_ENABLED) $(GO) build -o $(BINDIR)/ -ldflags "$(LDFLAGS)" $(GO_EXTRA_FLAGS) $(PKGDIR)/...
 
-.PHONY: docker-build-push
-docker-build-push:
-	@$(LOG) info "Building and push docker image"
+.PHONY: docker-build
+docker-build:
+	@$(LOG) info "Building docker image"
 	DOCKER_BUILDKIT=1 docker build -t "jaredallard/localizer:latest" .
 
 .PHONY: fmt