docs: update README Docker section

[MikeMcC399]

Issue

1. The GitHub Markdown extension: Alert [!NOTE] is rendered literally in the README as displayed on the npm registry instead of being interpreted.
2. Use of Docker working directory /e2e has been changed to the neutral /app directory in the examples in the https://github.com/cypress-io/cypress-docker-images repo.
3. The bookmark https://github.com/cypress-io/cypress-docker-images/blob/master/README.md#tag-selection was changed to #tags

Change

1. Replace [!NOTE] with the simple text NOTE: so that it renders correctly on the npm registry.
2. Replace the Docker working directory /e2e with the neutral /app directory in the examples.
3. Replace the bookmark https://github.com/cypress-io/cypress-docker-images/blob/master/README.md#tag-selection using the bookmark #tags.

[cypress-app-bot]

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