Fix Dockerfile.dev

home-assistant#34654 broke the dev dockerfile, so I'm trying to fix it.

- fix the leading && by putting a valid no-op command before it to maintain formatting
- remove the apt-add-repository, it's unclear why it was added and it's broken.
- also add a CI step that tries to build the dockerfile to validate it.

Author: jbergler

Dockerfile.dev

@@ -1,10 +1,10 @@
 FROM python:3.8
 
-RUN \
+RUN true \
     && apt-get update && apt-get install -y --no-install-recommends \
         software-properties-common \
-    && add-apt-repository ppa:jonathonf/ffmpeg-4 \
-    && apt-get update && apt-get install -y --no-install-recommends \
+    && apt-get update \
+    && apt-get install -y --no-install-recommends \
         libudev-dev \
         libavformat-dev \
         libavcodec-dev \

azure-pipelines-ci.yml

@@ -121,7 +121,16 @@ stages:
               . venv/bin/activate
               pre-commit run black --all-files --show-diff-on-failure
             displayName: "Check Black formatting"
-
+      - job: "ValidateDockerfiles"
+        pool:
+          vmImage: "ubuntu-latest"
+        steps:
+          - task: Docker@2
+            displayName: Build Dockerfile.dev
+            inputs:
+              repository: homeassistant-dev
+              command: build
+              Dockerfile: Dockerfile.dev
   - stage: "Tests"
     dependsOn:
       - "Overview"