chore: Add Dockerfile for convenient building (#4431)

This enables cases like `docker build --output=bin/
https://github.com/Azure/aks-engine.git` without having to manually
check out code and everything.
It supports cross compilation so a user can set
`--platform=darwin/amd64` or `--platform=local`, etc. to get a binary
for whatever they need.

Dockerfile

@@ -0,0 +1,33 @@
+# syntax=docker/dockerfile:1.2
+
+ARG GO_VERSION=1.16
+ARG GO_IMAGE=golang:${GO_VERSION}
+
+
+FROM --platform=$BUILDPLATFORM ${GO_IMAGE} AS build
+COPY . /go/src/github.com/azure/aks-engine
+WORKDIR /go/src/github.com/azure/aks-engine
+ARG TARGETARCH
+ARG TARGETOS
+ARG TARGETVARIANT
+SHELL ["/bin/bash", "-exc"]
+ENV GOCACHE=/go/.buildcache
+# We need to first generate everything w/o setting GOOS/GOARCH/GOARM
+# Otherwise it will try to build supporting binaries (for the build itself) for the $TARGETPLATFORM, which we don't want.
+# Those helper bins need to be for the $BUILDPLATFORM.
+RUN \
+	--mount=type=cache,target=/go/pkg/mod \
+	--mount=type=cache,target=/go/.buildcache \
+	make clean generate; \
+	export GOOS="$TARGETOS"; \
+	export GOARCH="$TARGETARCH"; \
+	if [ -n TARGETVARIANT ] && [ "$TARGETARCH" = "arm" ]; then \
+		export GOARM="${TARGETVARIANT//v}"; \
+	fi; \
+	make build
+
+
+# Alaways last so a simple `docker build` with no `--target` will produce the binary
+FROM scratch AS binary
+COPY --from=build /go/src/github.com/azure/aks-engine/bin/* /
+