Improved instructions & docker permissions fix

[Souvlaki42]

This PR is a direct consequence of these two #1172 and #1173.
It changes the following:

• Fixes some weird permission issues in the Dockerfile that runs with compiler/build.sh.
• Adds bash syntax highlighting in the shell code blocks in README.md.
• Adds markdown highlights in README.md to improve readability.
• Adds a troubleshoot in ui/README.md about having to set the server's address to 0.0.0.0 to expose it to the world.
• Adds a section in README.md that highlights:

Important

When running the server, you may see some docker warnings about missing containers. You can safely ignore them. You shouldn't see any docker containers running most of the time. This is normal. You don't need to spawn or kill any containers yourself. The backend manages that itself.