Publish arm image too (#21)

base image (https://hub.docker.com/_/docker/tags?page=1&name=20.10-dind) do support amd64 and arm64

so we can do too

Reviewed-on: https://codeberg.org/woodpecker-plugins/plugin-docker-buildx/pulls/21

Makefile

@@ -0,0 +1,6 @@
+TARGETOS ?= linux
+TARGETARCH ?= amd64
+LDFLAGS := -s -w -extldflags "-static"
+
+build:
+	CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -ldflags '${LDFLAGS}' -v -a -tags netgo -o docker-buildx ./cmd/docker-buildx