This is implemented as a state machine parser so it can emit sensible
error messages.
It does not use the connection strings elsewhere in rclone yet - see
subsequent commits.
An optional fuzzer is implemented for the Parse function.
Currently if container under test has multiple IP addresses,
the `docker_ip` function from `docker.sh` will return a gibberish.
This patch makes it return the first address found.
Additionally, I apply shellcheck on `docker.sh`.
