Dockerfile: install into site-library

This is a pattern that works if the package has any
dependencies.

[ci skip]

tools/docker/Dockerfile

@@ -2,11 +2,13 @@ FROM ghcr.io/r-hub/r-minimal/r-minimal:release AS build
 
 COPY . /root/nanoparquet
 
+RUN mkdir -p /usr/local/lib/R/site-library
+
 RUN installr -d local::/root/nanoparquet
 
 FROM ghcr.io/r-hub/r-minimal/r-minimal:release AS final
 
-COPY --from=build /usr/local/lib/R/library/nanoparquet /usr/local/lib/R/library/nanoparquet
+COPY --from=build /usr/local/lib/R/site-library /usr/local/lib/R/site-library
 
 RUN R -q -e 'library(nanoparquet); write_parquet(mtcars, "/tmp/mtcars.parquet"); read_parquet("/tmp/mtcars.parquet")' && \
     rm -rf /tmp/Rtmp*