fix: upload from local image generates broken command (#98)

While adding support for qcow2 images in #69 I broke support for local
images. Building a shell pipeline through string concatenation is not a
good idea...

The specific issue was fixed and I also moved building the shell
pipeline to a separate function and added unit tests for all cases, so
it should be easier to spot these issues in the future.

Closes #97

hcloudimages/doc_test.go

@@ -0,0 +1,33 @@
+package hcloudimages_test
+
+import (
+	"context"
+	"fmt"
+	"net/url"
+
+	"github.com/hetznercloud/hcloud-go/v2/hcloud"
+
+	"github.com/apricote/hcloud-upload-image/hcloudimages"
+)
+
+func ExampleClient_Upload() {
+	client := hcloudimages.NewClient(
+		hcloud.NewClient(hcloud.WithToken("<your token>")),
+	)
+
+	imageURL, err := url.Parse("https://example.com/disk-image.raw.bz2")
+	if err != nil {
+		panic(err)
+	}
+
+	image, err := client.Upload(context.TODO(), hcloudimages.UploadOptions{
+		ImageURL:         imageURL,
+		ImageCompression: hcloudimages.CompressionBZ2,
+		Architecture:     hcloud.ArchitectureX86,
+	})
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Printf("Uploaded Image: %d", image.ID)
+}