envbuilder: use container entrypoint if ENVBUILDER_INIT_SCRIPT is not set

By default, envbuilder runs `/bin/sh -c sleep infinity` as the entrypoint.
This behaviour can be surprising, so we should default to the entrypoint defined in the container if `ENVBUILDER_INIT_SCRIPT` / `ENVBUILDER_INIT_COMMAND` are not set.

Start by locating the handling of ENVBUILDER_INIT_SCRIPT and ENVBUILDER_INIT_COMMAND, then trace where the default `/bin/sh -c sleep infinity` entrypoint is selected. Verify that the container-defined entrypoint is used when both variables are unset, while the existing environment-variable overrides still work.

索引モデルが issue の本文から書いたものです。