Support podman --remote when Containerfile is not in context directory

Fixes: https://github.com/containers/podman/issues/18239

[NO NEW TESTS NEEDED]

@test "podman build -f test" in test/system/070-build.bats

Will test this.  This was passing when run on a local system since
the remote end was using the clients path to read the Containerfile
The issue is it would not work in a podman machine since the
Containerfile would/should be a different path.

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

pkg/api/handlers/compat/images_build.go

@@ -221,9 +221,11 @@ func BuildImage(w http.ResponseWriter, r *http.Request) {
 			var m = []string{}
 			if err := json.Unmarshal([]byte(query.Dockerfile), &m); err != nil {
 				// it's not json, assume just a string
-				m = []string{filepath.Join(contextDirectory, query.Dockerfile)}
+				m = []string{query.Dockerfile}
+			}
+			for _, containerfile := range m {
+				containerFiles = append(containerFiles, filepath.Join(contextDirectory, filepath.Clean(filepath.FromSlash(containerfile))))
 			}
-			containerFiles = m
 			dockerFileSet = true
 		}
 	}