compat handlers: add X-Registry-Auth header support

* Support the `X-Registry-Auth` http-request header.

 * The content of the header is a base64 encoded JSON payload which can
   either be a single auth config or a map of auth configs (user+pw or
   token) with the corresponding registries being the keys.  Vanilla
   Docker, projectatomic Docker and the bindings are transparantly
   supported.

 * Add a hidden `--registries-conf` flag.  Buildah exposes the same
   flag, mostly for testing purposes.

 * Do all credential parsing in the client (i.e., `cmd/podman`) pass
   the username and password in the backend instead of unparsed
   credentials.

 * Add a `pkg/auth` which handles most of the heavy lifting.

 * Go through the authentication-handling code of most commands, bindings
   and endpoints.  Migrate them to the new code and fix issues as seen.
   A final evaluation and more tests is still required *after* this
   change.

 * The manifest-push endpoint is missing certain parameters and should
   use the ABI function instead.  Adding auth-support isn't really
   possible without these parts working.

 * The container commands and endpoints (i.e., create and run) have not
   been changed yet.  The APIs don't yet account for the authfile.

 * Add authentication tests to `pkg/bindings`.

Fixes: #6384
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

libpod/runtime.go

@@ -339,9 +339,10 @@ func makeRuntime(ctx context.Context, runtime *Runtime) (err error) {
 	}
 
 	// Set up containers/image
-	runtime.imageContext = &types.SystemContext{
-		SignaturePolicyPath: runtime.config.Engine.SignaturePolicyPath,
+	if runtime.imageContext == nil {
+		runtime.imageContext = &types.SystemContext{}
 	}
+	runtime.imageContext.SignaturePolicyPath = runtime.config.Engine.SignaturePolicyPath
 
 	// Create the tmpDir
 	if err := os.MkdirAll(runtime.config.Engine.TmpDir, 0751); err != nil {