prompt-less signing via passphrase file

To support signing images without prompting the user, add CLI flags for
providing a passphrase file.

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

vendor/github.com/proglottis/gpgme/unset_agent_info_windows.go

@@ -0,0 +1,14 @@
+package gpgme
+
+// #include <stdlib.h>
+import "C"
+import (
+	"unsafe"
+)
+
+// unsetenv is not available in mingw
+func unsetenvGPGAgentInfo() {
+	v := C.CString("GPG_AGENT_INFO=")
+	defer C.free(unsafe.Pointer(v))
+	C.putenv(v)
+}