cmd/server: Die when system secret is in wrong format

[aeneasr]

Follow up to #816

[nicerobot]

Maybe this is also partly a documentation enhancement:

$ export SYSTEM_SECRET=this_needs_to_be_the_same_always_and_also_very_$3cuR3-._

is fine to explain the variable but we could also recommend a fast way to obtain a relatively secure secret, e.g.:

$ export SYSTEM_SECRET=$(export LC_CTYPE=C; cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 32 | head -n 1)

The key here being that I was aware that I would need to retain this secret but I didn't catch that the first time I tried, the secret I used was ignored and a random one was generated. So when I tried restarting with the same insecure secret, it caused the failure which was unexpected.

[aeneasr]

Yeah, that's an awesome suggestion @nicerobot ! Got some unix swag! You don't know - by any chance - the equivalent for windows?

[nicerobot]

@arekkas I don't but i can look into it.

[nicerobot]

@arekkas I've had it tested in the Windows 10 Linux Subsystem and that does work. Now, figuring out a solution for Windows classic shell is another story.

[aeneasr]

Tracked as ory/docs#27