wip: Fix docker build

Dockerfile

@@ -1,32 +1,37 @@
 # Build the manager binary
-FROM golang:1.17 as builder
-
-ARG TARGETPLATFORM
+FROM --platform=$BUILDPLATFORM golang:1.17 as builder
 
 WORKDIR /workspace
 
-ENV GO111MODULE=on \
-  CGO_ENABLED=0
-
-# # Copy the Go Modules manifests
-# COPY go.mod go.sum ./
+# Copy the Go Modules manifests
+COPY go.mod 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 --mount=type=cache,target=/go/pkg/mod go mod download
+# 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.
+#
+# Also, we need to do this before setting TARGETPLATFORM/TARGETOS/TARGETARCH/TARGETVARIANT
+# so that go mod cache is shared across platforms.
+RUN go mod download
 
 # Copy the go source
 # COPY . .
 
-ARG TARGETOS
-ARG TARGETARCH
+# Usage:
+#   docker buildx build --tag repo/img:tag -f ./Dockerfile . --platform linux/amd64,linux/arm64,linux/arm/v7
+#
+# With the above commmand,
+# TARGETOS can be "linux", TARGETARCH can be "amd64", "arm64", and "arm", TARGETVARIANT can be "v7".
+
+ARG TARGETPLATFORM TARGETOS TARGETARCH TARGETVARIANT
+
+# We intentionally avoid `--mount=type=cache,mode=0777,target=/go/pkg/mod` in the `go mod download` and the `go build` runs
+# to avoid https://github.com/moby/buildkit/issues/2334
+# We can use docker layer cache so the build is fast enogh anyway
 
 # Build
 RUN --mount=target=. \
   --mount=type=cache,mode=0777,target=/root/.cache/go-build \
-  --mount=type=cache,mode=0777,target=/go/pkg/mod\
-  GOOS=${TARGETOS} GOARCH=${TARGETARCH} \
-  GOARM=$(echo ${TARGETPLATFORM} | cut -d / -f3 | cut -c2-) \
+  GOOS=${TARGETOS} GOARCH=${TARGETARCH} GOARM=${TARGETVARIANT#v} \
   go build -o /out/manager main.go && go build -o /out/github-webhook-server ./cmd/githubwebhookserver
 
 # Use distroless as minimal base image to package the manager binary