bib: simplify getContainerSize()

The getContainerSize() was not using some of the modern go helpers.
So let's use `exec.Command().Output()` and introduce a new
`util.OutputErr()` helper that will be able to also show stderr to
the user if the Output() call returns an error.

bib/internal/util/util.go

@@ -26,3 +26,12 @@ func RunCmdSync(cmdName string, args ...string) error {
 	}
 	return nil
 }
+
+// OutputErr takes an error from exec.Command().Output() and tries
+// generate an error with stderr details
+func OutputErr(err error) error {
+	if err, ok := err.(*exec.ExitError); ok {
+		return fmt.Errorf("%w, stderr:\n%s", err, err.Stderr)
+	}
+	return err
+}

bib/internal/util/util_test.go

@@ -0,0 +1,21 @@
+package util_test
+
+import (
+	"fmt"
+	"os/exec"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/osbuild/bootc-image-builder/bib/internal/util"
+)
+
+func TestOutputErrPassthrough(t *testing.T) {
+	err := fmt.Errorf("boom")
+	assert.Equal(t, util.OutputErr(err), err)
+}
+
+func TestOutputErrExecError(t *testing.T) {
+	_, err := exec.Command("bash", "-c", ">&2 echo some-stderr; exit 1").Output()
+	assert.Equal(t, "exit status 1, stderr:\nsome-stderr\n", util.OutputErr(err).Error())
+}