Fix nginx 413 error

[nlgotz]

Description
When dealing with large alerts (>1M) NGINX would return a 413 error. This fixes that by allowing the end user to set the NGINX client_max_body_size via the environment variable NGINX_MAX_BODY_SIZE. NGINX defaults to 1M which was causing issues with large alerts.

Fixes #441

Changes
Include a brief summary of changes...

• added environment variable to nginx.conf.j2
• added sample configuration to docker-compose.yml (commented out by default)

Checklist

• Pull request is limited to a single purpose
• Code style/formatting is consistent
• All existing tests are passing
• Added new tests related to change
• No unnecessary whitespace changes

Collaboration
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generally has the authority to decide if other users can commit to the
pull request's compare branch. If the pull request author wants greater
collaboration, they can grant maintainers of the upstream repository
(that is, anyone with push access to the upstream repository) permission
to commit to the pull request's compare branch

See https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/allowing-changes-to-a-pull-request-branch-created-from-a-fork