Add app container

[Scotchester]

⚠️ base branch: feature/slack-app-code ⚠️

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PR 2 of ??

This PR adds:

• appcontainer/Dockerfile
• an entry for it in compose.yml
• .dockerignore
• .env.sample

Testing

After completing the basic Slack app setup (see #1), the application can now be run in Docker:

1. cp .env.sample .env and put your tokens in there
2. docker compose up app

You should see output indicating that the app is now running, and you should now be able to invoke it in your Slack test workspace! Try the /define command to confirm.

[Scotchester]

My general approach here was to start from scratch with the official python:3.14-alpine image, copy in the parts of the Benefits app container Dockerfile that seemed relevant for this much simpler app (cross-referencing the docker-python-web Dockerfile for good measure), and then update it for the bits where Alpine demands different approaches (user/group creation, installing Bash).