Add dockerignore tester to xonsh config

xonsh/docker.xsh

@@ -0,0 +1,34 @@
+_DOCKERFILE_TEXT = """
+FROM busybox
+COPY . /build-context
+WORKDIR /build-context
+CMD find .
+"""
+
+def _dockerignore_check(args):
+    if len(args) != 1:
+        print("Must specify only one argument: The dockerignore file.")
+        return
+
+    ignore_file = Path(args[0])
+
+    if not ignore_file.exists():
+        print(f"Dockerignore file {ignore_file} doesn't exist.")
+
+    import tempfile
+    import shutil
+
+    with tempfile.TemporaryDirectory() as td:
+        dpath = Path(td)
+
+        docker_path = dpath / "Dockerfile"
+        ignore_path = dpath / "Dockerfile.dockerignore"
+        docker_path.write_text(_DOCKERFILE_TEXT)
+        shutil.copyfile(ignore_file, ignore_path)
+
+        $DOCKER_BUILDKIT=1
+        docker image build --no-cache -t dockerignore-build-context -f @(docker_path) . &>/dev/null
+        docker run -it --rm dockerignore-build-context
+        docker image rm dockerignore-build-context &> /dev/null
+
+aliases["ditest"] = _dockerignore_check