Replace the cp from busybox

Use a simple program to copy the plugin binary, so that we will not depend on
busybox.

Signed-off-by: Daniel Jiang <jiangd@vmware.com>

Dockerfile

@@ -27,12 +27,11 @@ ENV GOOS=${TARGETOS} \
 COPY . /go/src/velero-plugin-for-microsoft-azure
 WORKDIR /go/src/velero-plugin-for-microsoft-azure
 RUN export GOARM=$( echo "${GOARM}" | cut -c2-) && \
-    CGO_ENABLED=0 go build -v -o /go/bin/velero-plugin-for-microsoft-azure ./velero-plugin-for-microsoft-azure
-
-FROM busybox@sha256:91540637a8c1bd8374832a77bb11ec286c9599ff8b528d69794f5dea6e257fd9 AS busybox
+    CGO_ENABLED=0 go build -v -o /go/bin/velero-plugin-for-microsoft-azure ./velero-plugin-for-microsoft-azure && \
+    CGO_ENABLED=0 go build -v -o /go/bin/cp-plugin ./hack/cp-plugin
 
 FROM scratch
 COPY --from=build /go/bin/velero-plugin-for-microsoft-azure /plugins/
-COPY --from=busybox /bin/cp /bin/cp
+COPY --from=build /go/bin/cp-plugin /bin/cp-plugin
 USER 65532:65532
-ENTRYPOINT ["cp", "/plugins/velero-plugin-for-microsoft-azure", "/target/."]
+ENTRYPOINT ["cp", "/plugins/velero-plugin-for-microsoft-azure", "/target/velero-plugin-for-microsoft-azure"]

hack/cp-plugin/main.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+)
+
+func main() {
+	if len(os.Args) != 3 {
+		fmt.Println(
+			`Error: This command requires two arguments.
+Usage: cp-plugin src dst`)
+		os.Exit(1)
+	}
+	src, dst := os.Args[1], os.Args[2]
+	fmt.Printf("Copying %s to %s ...  ", src, dst)
+	srcFile, err := os.Open(src)
+	if err != nil {
+		panic(err)
+	}
+	defer srcFile.Close()
+	if _, err := os.Stat(dst); errors.Is(err, os.ErrNotExist) {
+		_, err = os.Create(dst)
+		if err != nil {
+			panic(err)
+		}
+	}
+	dstFile, err := os.OpenFile(dst, os.O_WRONLY, 0755)
+	if err != nil {
+		panic(err)
+	}
+	defer dstFile.Close()
+	buf := make([]byte, 1024*128)
+	_, err = io.CopyBuffer(dstFile, srcFile, buf)
+	if err != nil {
+		panic(err)
+	}
+	os.Chmod(dst, 0755)
+	fmt.Println("done.")
+}