docker/README.md can lead to world-readable API key

[ElectricNroff]

This might be considered a #621 regression because the current version removes code that had been added in 21320be

https://github.com/CVEProject/cve-services/blob/7f683512e5bcd07f8ce48d41d06ea533c90dd265/docker/README.md
says

Create a curl config file in $HOME/.curl-cve-config similar to the following

but does not discuss whether file permissions should be set in any specific way. Many popular systems have a default configuration in which files in a user's home directory can be read by some or all other users. There is a risk that an API key could be stolen if a threat actor has unprivileged local access to a host that is used for command-line CVE Services API access.

By contrast, https://github.com/CVEProject/cve-services/blob/6e4f176ff305ceb0e7747d2e5991a580267de73f/docker/README.md had set the .curl-cve-config file permissions to 600 before writing the API key into the file.

[slubar]

Add instructions to set the .curl-cve-config file permissions to 600 before writing the API key into the file.