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.go

@@ -0,0 +1,18 @@
+// +build !windows
+
+package gpgme
+
+// #include <stdlib.h>
+import "C"
+import (
+	"unsafe"
+)
+
+// This is somewhat of a horrible hack. We need to unset GPG_AGENT_INFO so that gpgme does not pass --use-agent to GPG.
+// os.Unsetenv should be enough, but that only calls the underlying C library (which gpgme uses) if cgo is involved
+// - and cgo can't be used in tests. So, provide this helper for test initialization.
+func unsetenvGPGAgentInfo() {
+	v := C.CString("GPG_AGENT_INFO")
+	defer C.free(unsafe.Pointer(v))
+	C.unsetenv(v)
+}