more idiomatic setup for Dockerfile

.devcontainer/Dockerfile

@@ -0,0 +1,7 @@
+FROM python:3.9-buster
+
+COPY requirements.txt ./
+RUN pip install --no-cache-dir -r requirements.txt
+RUN pip install --no-cache-dir black
+COPY . .
+

.devcontainer/devcontainer.json

@@ -5,7 +5,7 @@
 	// Sets the run context to one level up instead of the .devcontainer folder.
 	"context": "..",
 	// Update the 'dockerFile' property if you aren't using the standard 'Dockerfile' filename.
-	"dockerFile": "../DockerDev",
+	"dockerFile": "Dockerfile",
 	// Set *default* container specific settings.json values on container create.
 	"settings": {},
 	// Add the IDs of extensions you want installed when the container is created.