Enable ARM64 linux builds.

BUILDING.md

@@ -11,25 +11,33 @@ All code under the `pkg/` directory should be thoroughly tested.
 
 ## Building binary
 
-Use `make test` to build all platofrm binaries to the `bin/` directory.
+Use `make build` to build all platofrm binaries to the `bin/` directory.
 
 Mac:
 
-```
+```bash
 # builds to bin/darwin/amd64/oras
 make build-mac
 ```
 
 Linux:
 
+```bash
+# builds to bin/linux/amd64/oras
+make build-linux
 ```
+
+Linux  ARM64:
+
+```bash
 # builds to bin/linux/amd64/oras
 make build-linux
 ```
 
+
 Windows:
 
-```
+```bash
 # builds to bin/windows/amd64/oras.exe
 make build-windows
 ```

Makefile

@@ -28,13 +28,18 @@ clean:
 	git status --ignored --short | grep '^!! ' | sed 's/!! //' | xargs rm -rf
 
 .PHONY: build
-build: build-linux build-mac build-windows
+build: build-linux build-linux-arm64 build-mac build-windows
 
 .PHONY: build-linux
 build-linux:
 	GOARCH=amd64 CGO_ENABLED=0 GOOS=linux go build -v --ldflags="$(LDFLAGS)" \
 		-o bin/linux/amd64/$(CLI_EXE) $(CLI_PKG)
 
+.PHONY: build-linux-arm64
+build-linux-arm64:
+	GOARCH=arm64 CGO_ENABLED=0 GOOS=linux go build -v --ldflags="$(LDFLAGS)" \
+		-o bin/linux/arm64/$(CLI_EXE) $(CLI_PKG)
+
 .PHONY: build-mac
 build-mac:
 	GOARCH=amd64 CGO_ENABLED=0 GOOS=darwin go build -v --ldflags="$(LDFLAGS)" \