Further improve our docker image.

[myronmarston]

• Fix the docker ignores. They weren't working at all; apparently .dockerignore has to be at the content root (which is our repository root). I've also streamlined the .dockerignore file itself.
• Avoid log messages that say "fatal: not a git repository (or any of the parent directories): .git".
• Avoid a docker networking warning¹.

Footnotes

1. WARN[0215] a network with name opensearch exists but was not created for project "docker_demo". Set external: true to use an existing network ↩

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