This change will only copy the `compose.{json|lock}` files prior to
installing the dependencies. This improves container image build time
when only code changed, as it facilitates image caching.
This is related to issues with iconv on alpine linux (see [1]). I
noticed that the content file `tipuesearch_content.json` didn't contain
the sanitized text, but only the boolean `false`. I tracked it down to
the `iconv` call. After migration to the debian docker image it is
working fine. Build was started using `docker run --rm -it -w /build -v
"$PWD":/build daux/daux.io daux generate`.
[1] https://github.com/docker-library/php/issues/428
