versioning: Set DEFAULT_DOCKCROSS_IMAGE based on the image tag

This ensures that the dockcross script will continue to use the specific
tagged version of the image that generates it.

android-arm64/Dockerfile

@@ -39,18 +39,19 @@ RUN mkdir -p /build && \
     find ${CROSS_ROOT} -executable -exec chmod a+x '{}' \;
 
 
-ENV DEFAULT_DOCKCROSS_IMAGE dockcross/android-arm64
-
 COPY Toolchain.cmake ${CROSS_ROOT}/
 ENV CMAKE_TOOLCHAIN_FILE ${CROSS_ROOT}/Toolchain.cmake
 
 # Build-time metadata as defined at http://label-schema.org
 ARG BUILD_DATE
-ARG IMAGE
+ARG IMAGE=dockcross/android-arm64
+ARG VERSION=latest
 ARG VCS_REF
 ARG VCS_URL
 LABEL org.label-schema.build-date=$BUILD_DATE \
       org.label-schema.name=$IMAGE \
+      org.label-schema.version=$VERSION \
       org.label-schema.vcs-ref=$VCS_REF \
       org.label-schema.vcs-url=$VCS_URL \
       org.label-schema.schema-version="1.0"
+ENV DEFAULT_DOCKCROSS_IMAGE ${IMAGE}:${VERSION}