Dockerfile: include spdx-licenses-list

This is for linting license field on Travis.

One can also customize image as simple as
docker build "--build-arg=ADDINS=moreutils jq"

Dockerfile

@@ -17,6 +17,7 @@ FROM scratch as stage1
 ARG REPOSITORY=https://alpha.de.repo.voidlinux.org
 ARG ARCH=x86_64
 ARG BASEPKG=base-minimal
+ARG ADDINS=
 COPY --from=stage0 /target /
 COPY keys/* /target/var/db/xbps/keys/
 RUN xbps-reconfigure -a && \
@@ -25,7 +26,7 @@ RUN xbps-reconfigure -a && \
     --repository=${REPOSITORY}/current \
     --repository=${REPOSITORY}/current/musl \
     -r /target \
-    ${BASEPKG}
+    ${BASEPKG} ${ADDINS}
 
 # 3) configure and clean up the final image
 FROM scratch