Using double quotes when specifying `BORG_SERVE_ARGS` causes incorrect authorized keys generation

[SolidEva]

setting

  environment:
   - BORG_SERVE_ARGS="--storage-quota 1.5T"

results in the generation of the following authorized keys line:

restrict,command="cd /backup/<hostname>; borg serve --restrict-to-path /backup/<hostname> "--storage-quota 1.5T"" <key-redacted>

which is not a valid authorized keys file

borgserver-1  |  * Validating structure of generated /home/borg/.ssh/authorized_keys...
borgserver-1  | ERROR: /home/borg/.ssh/authorized_keys is not a public key file.

the easy solution is to either use single quotes, or no quotes instead:

  environment:
   - BORG_SERVE_ARGS=--storage-quota 1.5T

but perhaps we should detect when double quotes are used, and error out in a more helpful way.