Add a way to inject environment variables for deployment

[martin-helmich]

Status quo:

• Write an .env file in a previous action step, and have this action pick that file up

Ideas:

• Just pass through all environment variables of the action. Probably a very bad idea.

• Add an environment input to the action:

  - name: Deploy to Mittwald Container Hosting
    uses: mittwald/rp-container-deploy-action@master
    with:
      mittwald-api-token: ${{ secrets.MITTWALD_API_TOKEN }}
      mittwald-project-id: 'p-XXXXXX'
      environment: |
        FOO=BAR
        BAR=${{ secrets.BAR_SECRET }}