Fix missing CGO_EXTRA_FLAGS build arg for docker (#11782)

Co-authored-by: zeripath <art27@cantab.net>

Dockerfile

@@ -9,6 +9,7 @@ ENV GOPROXY ${GOPROXY:-direct}
 ARG GITEA_VERSION
 ARG TAGS="sqlite sqlite_unlock_notify"
 ENV TAGS "bindata $TAGS"
+ARG CGO_EXTRA_CFLAGS
 
 #Build deps
 RUN apk --no-cache add build-base git nodejs npm