Add Bitbucket CI instructions (#93)

    expire_in: 2 week

```

### How do I use image in Bitbucket CI?

[Bitbucket doesn't support custom entrypoints](https://confluence.atlassian.com/bbkb/bitbucket-pipelines-does-not-execute-the-build-image-s-entrypoint-cmd-script-1299910012.html),

so we need to manually call it. Otherwise the setup is similar to what you would

do with GitLab CI.

```yaml

pipelines:

  default:

    - step:

        name: Build Windows executable

        image: batonogov/pyinstaller-windows:latest

        artifacts:

          paths:

            - *.exe

        script:

          - echo "Creating Windows artifact"

          - SPECFILE="$BITBUCKET_CLONE_DIR/sparvio-logger-bootstrap.spec" WORKDIR="$BITBUCKET_CLONE_DIR" bash /entrypoint.sh

          - pip install -r ./test/requirements.txt

          - cd ./test && pyinstaller --onefile main.py

          - cp ./dist/*.exe ../

```

## Known Issues

[ntdll.so Path Missing](https://github.com/batonogov/docker-pyinstaller/issues/23)