Add 17.09 archive to Dockerfile (docker#5660)

Dockerfile

@@ -78,6 +78,7 @@ COPY --from=docs/docker.github.io:v1.12 ${TARGET} ${TARGET}
 COPY --from=docs/docker.github.io:v1.13 ${TARGET} ${TARGET}
 COPY --from=docs/docker.github.io:v17.03 ${TARGET} ${TARGET}
 COPY --from=docs/docker.github.io:v17.06 ${TARGET} ${TARGET}
+COPY --from=docs/docker.github.io:v17.09 ${TARGET} ${TARGET}
 
 # Get the built docs output from the previous build stage
 # This ordering means all previous layers can come from cache unless an archive