add optional verbose ENV about docker-shadowsocks-libev

[vimagick]

I'm going to add general OPTIONS to allow user pass any options.

CMD ss-server -s "$SERVER_ADDR" \
              -p "$SERVER_PORT" \
              -m "$METHOD" \
              -k "$PASSWORD" \
              -t "$TIMEOUT" \
              -d "$DNS_ADDR" \
              -u \
              -A \
              --fast-open $OPTIONS

I need to test the behavior of empty variable.

[vimagick]

You can pass multiple options (without single/double quotes):

  environment:
    - OPTIONS=-v --mptcp