remove unused archs

I am not using nor need arm64 and windows. Removing these from the
Makefile. If you see this message and feel like they should be added
back (or different archs), ping me.

Makefile

@@ -1,30 +1,27 @@
 SCRIPTS = $(shell awk '/#!\/bin\/(ba)?sh/&&FNR==1{print FILENAME}' $(shell git ls-files))
 GODEPS = $(shell git ls-files '*.go' go.mod go.sum)
 GOBIN = $(shell go env GOPATH)/bin/
-
-GOOSARCHS = $(sort \
-			darwin/amd64 \
-			darwin/arm64 \
-			linux/amd64 \
-			linux/arm64 \
-			windows/amd64/.exe)
+GOOSARCHS = $(sort darwin/amd64 linux/amd64)
 
 VSN ?= $(shell git describe --dirty)
 VSNHASH = $(shell git rev-parse --verify HEAD)
 LDFLAGS = -ldflags "-X main.Version=$(VSN) -X main.VersionHash=$(VSNHASH)"
 
+undocker: ## builds binary for the current architecture
+	CGO_ENABLED=0 go build $(LDFLAGS) -o $@
+
 .PHONY: test
 test:
 	go test -race -cover ./...
 
 define undockertarget
-UNDOCKERS += undocker-$(1)-$(2)-$(VSN)$(firstword $(3))
-undocker-$(1)-$(2)-$(VSN)$(firstword $(3)): $(GODEPS)
+UNDOCKERS += undocker-$(1)-$(2)-$(VSN)
+undocker-$(1)-$(2)-$(VSN): $(GODEPS)
 	CGO_ENABLED=0 GOOS=$(1) GOARCH=$(2) go build $(LDFLAGS) -o $$@
 endef
 
 $(foreach goosarch,$(GOOSARCHS),\
-	$(eval $(call undockertarget,$(word 1,$(subst /, ,$(goosarch))),$(word 2,$(subst /, ,$(goosarch))),$(word 3,$(subst /, ,$(goosarch))))))
+	$(eval $(call undockertarget,$(word 1,$(subst /, ,$(goosarch))),$(word 2,$(subst /, ,$(goosarch))))))
 
 .PHONY: all
 all: $(UNDOCKERS)

README.md

@@ -20,6 +20,19 @@ systemd, FreeBSD Jails, and many others.
 
 Undocker has no dependencies outside Golang stdlib.
 
+Installation
+------------
+
+We recommend using [officially released binaries][3]. To build the project
+instead, run:
+
+```
+$ make undocker
+```
+
+The number of officially released binaries is quite limited. If you'd like me
+to expand a list, please contribute a patch to the Makefile.
+
 Usage: convert docker image to rootfs
 -------------------------------------
 
@@ -107,3 +120,4 @@ MIT
 
 [1]: https://www.freedesktop.org/software/systemd/man/systemd.exec.html
 [2]: https://fly.io/blog/docker-without-docker/
+[3]: http://git.sr.ht/~motiejus/undocker