Add support for SCTP in EXPOSE instruction

[rcjsuen]

In Docker CE 18.03, they added support for exposing SCTP ports.

FROM scratch
EXPOSE 8080/sctp

$ docker build .
Sending build context to Docker daemon  1.775MB
Step 1/2 : FROM scratch
 --->
Step 2/2 : EXPOSE 9090/sctp
 ---> Running in 2edd97fe038c
Removing intermediate container 2edd97fe038c
 ---> 449d46ebd47c
Successfully built 449d46ebd47c

We need to add support for this too as it's currently flagged as an error.

$ dockerfile-utils lint Dockerfile
Line: 2
EXPOSE 9090/sctp
            ^^^^
Error: Invalid proto: sctp