mico-admin entrypoint: avoid truncating nginx.conf

[s1mn]

Description

Rather than trying to operate on the nginx.conf file in place (see below on why this fails), this PR changes the docker entrypoint to create a temporary file and then replace nginx.conf with that one.

There are no breaking changes in this PR.

Resolves / is related to

Currently, the mico-admin docker entrypoint reads and writes the nginx.conf in the same shell command. This is a race condition that often leads to erasing the contents of the nginx.conf, because

• the file is opened for reading (fine)
• the file is opened for writing and truncated so it's empty
• then envsubst tries to do its job, but ends up doing nothing because the file it's trying to read is empty

The shellcheck wiki has an article about this class of problems: https://github.com/koalaman/shellcheck/wiki/SC2094

What is affected by this PR

• [x] mico-admin Docker image requires a rebuild
• [x] Backend
  • [ ] Kubernetes logic
  • [ ] Domain model
• [ ] API
  • [ ] discussed changes in the team / informed the team about changes
  • [ ] updated insertTestValues.sh and Postman Collections are updated
• [ ] Frontend
  • [ ] Ensure that it is compilable for production (npm run build -- --prod)
• [ ] Documentation

Checklist

• (not applicable: no new files) Ensure that new source files include the license header
• (not applicable: just a bugfix, no changes necessary) Ensure JavaDoc is up to date