Generate assets at the image build time

[bqbn]

Describe the problem and steps to reproduce it:

Similar to https://github.com/mozilla/addons/issues/2184, we'd like to request that addons-code-manager to run yarn build at the image build time.

This will reduce the app start time and additionally, we can remove the repetitive work of building the assets for each container.

For reference, the following env variables are passed to the app when the container starts,

NODE_ENV=production
SERVER_HOST=0.0.0.0
SERVER_PORT=4000
REACT_APP_API_HOST=https://addons.mozilla.org
PUBLIC_URL=https://addons-code-manager.cdn.mozilla.net
REACT_APP_REVIEWERS_HOST=https://reviewers.addons.mozilla.org

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