main: check if local storage has been mounted

Since we are using local storage by default, Wwe should fail early if it
hasn't been mounted. Some undefined behaviour has been seen when local storage
volume hasn't been mounted and the `--local` flag has been provided.

bib/cmd/bootc-image-builder/main.go

@@ -12,6 +12,7 @@ import (
 	"strings"
 
 	"github.com/osbuild/bootc-image-builder/bib/internal/setup"
+	"github.com/osbuild/bootc-image-builder/bib/internal/util"
 	"github.com/osbuild/images/pkg/arch"
 	"github.com/osbuild/images/pkg/blueprint"
 	"github.com/osbuild/images/pkg/cloud/awscloud"
@@ -29,9 +30,10 @@ import (
 var reposStr string
 
 const (
-	distroName       = "fedora-39"
-	modulePlatformID = "platform:f39"
-	releaseVersion   = "39"
+	distroName            = "fedora-39"
+	modulePlatformID      = "platform:f39"
+	releaseVersion        = "39"
+	containersStoragePath = "/var/lib/containers/storage"
 )
 
 type BuildConfig struct {
@@ -102,6 +104,12 @@ func loadConfig(path string) (*BuildConfig, error) {
 }
 
 func makeManifest(c *ManifestConfig, cacheRoot string) (manifest.OSBuildManifest, error) {
+	// if "/var/lib/containers/storage" hasn't been mounted and the `--local` flag has been provided,
+	// we should return an error. If it's not mounted, it can cause some undefined behaviour.
+	if c.Local && !util.IsMountpoint(containersStoragePath) {
+		return nil, fmt.Errorf("%s has not been mounted, but `--local` flag has been used", containersStoragePath)
+	}
+
 	// If --local wasn't given, always pull the container.
 	// If the user mount a container storage inside bib (without --local), the code will try to pull
 	// a newer version of the container even if an older one is already present. This doesn't match