Backport of #49612.
The current Docker entrypoint script picks up environment variables and
translates them into -E command line arguments. However, since any tool
executes via `docker exec` doesn't run the entrypoint, it results in
a poorer user experience.
Therefore, refactor the env var handling so that the -E options are
generated in `elasticsearch-env`. These have to be appended to any
existing command arguments, since some CLI tools have subcommands and
-E arguments must come after the subcommand.
Also extract the support for `_FILE` env vars into a separate script, so
that it can be called from more than once place (the behaviour is
idempotent).
Finally, add noop -E handling to CronEvalTool for parity, and support
`-E` in MultiCommand before subcommands.
Remove references to the `platinum` image and add a self-generated trial
licence to the example for TLS on Docker.
Fixeselastic/elasticsearch-docker#176
Rory Hunter
Lisa Cawley
Toby McLaughlin
Jason Tedor
Ryan Ernst