Dashboard docker path routing

[sentanos]

Description

The current docker image does not support any path routing. If you go to any
path other than the root, it 404s.

For example, if you go to a page other than the index and then refresh, it will
404.

This adds basic routing to fall back to index.html if a file isn't found. This
is already properly handled by the built index.html such that an invalid path
will show a proper 404 page (rather than the nginx 404 page).

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Note: make sure your files are formatted with rust-analyzer

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[sentanos]

An alternative would be #422, but I considered this a less obtrusive change.