This adds a `Dockerfile` so that the app can be built as a Docker image.

    docker build -t metacpan-api .

One can start a development server on Docker via

    docker run -p 5000:5000 -it metacpan-api

This is part of an effort to Dockerize MetaCPAN (including the web
frontend and ElasticSearch services.)

Docker encourages logging to STDOUT/STDERR (e.g in docker-compose) so
autoflush those when running in containers.

Reduce image disk usage by removing cpan/cpm caches as well as apt
package lists and whatever else in /tmp.

Since .dockerignore is symlinked to .gitignore, this also prevents
including the host Carton local installation into the Docker image.

We can have a separate ElasticSearch container for testing, so let it be
configurable via another variable so we can use both ES containers from
the same API container.

Copying in the repo is somewhat unneeded now since we'll use the image
primarily to provide a development container, and we can always mount in
the repo from the Docker host via `docker run -v` and Docker Compose
`volume`.

Slightly emphasize its a continuation of previous RUN subcommand.

It seems that Catalyst::Log is supposed to be able to autoflush by
default, but not in the case of Docker accepting logs via
STDOUT/STDERR.  Other Log objects like from Log::Dispatch or
Log::Log4Perl don't seem to have the same issue.

Dockerize the MetaCPAN