Installation documentation update

[Vankerkom]

During a manual setup I have encountered some missing information in the installation guide.
I have updated the instructions, fixed some bugs and added some extra information on things I discovered while setting up trace.moe locally.

Changes:

• Change public IPs to localhost since the internal docker IP does not work on Windows.
• Added missing TRACE_API_SALT in docker-compose.yaml to fix user creation.
• Added relative directories in the .env.example to be OS independent.
• Added more calrification when importing files:
  • Added the explicit directory name where files should be located.
  • Required extension is .mp4
• Changed the restart flags of containers to always be restart: unless-stopped

[soruly]

thank you <3