Fix Dockerfile.dev

[jbergler]

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

Proposed change

• 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.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

The integration reached or maintains the following Integration Quality Scale:

• No score or internal
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• 🥇 Gold
• 🏆 Platinum

[homeassistant]

Hi @jbergler,

It seems you haven't yet signed a CLA. Please do so here.

Once you do that we will be able to review and accept this pull request.

Thanks!

[pvizeli]

2cae17d

[pvizeli]

For dockerfile, hadolint is the feature: #34758

Yeah, the ffmpeg part was a copy past issue from our azure ci dockerfile