Restrain permissions on sensitive files

[buchdag]

This PR is a follow up to #412. It add checks and adjustment of ownership / permissions of sensitive files (private keys and ACME account keys) and enclosing folders + corresponding test unit.

The following files and folders are are currently adjusted by the PR:

[drwx------]  /etc/nginx/certs
├── [-rw-------]  default.key
├── [drwx------]  accounts
│   └── [drwx------]  acme-v01.api.letsencrypt.org
│       └── [drwx------]  directory
│           └── [-rw-------]  default.json
├── [drwx------]  domain.tld
│   └── [-rw-------]  key.pem
└── [drwx------]  subdomain.domain.tld
    └── [-rw-------]  key.pem

I'm not sure yet about changing /etc/nginx/certs ownership and permissions, I suspect it might break things for people using host volumes.

[buchdag]

@JrCs any advice on changing /etc/nginx/certs permissions vs leaving them as they are ?

It works in both case with docker volumes, but it might be an inconvenience for people using host volumes (ie -v /path/to/certs:/etc/nginx/certs ).

[buchdag]

I've decided not to touch /etc/nginx/certs ownership and permissions.

@JrCs can you review and approve ?

[buchdag]

ping @JrCs