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

    docker build -t metacpan-web .

One can start a development server on Docker via

    docker run -p 5001:5001 -it metacpan-web

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

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

It seems that Net::Async::HTTP clients with `max_connections_per_host >
1` are not friendly with single-threaded API backend (`plackup -s
Standalone`), so enable setting connection pooling from the environment
if possible.  Net::Async::HTTP already [documents][0] a
`NET_ASYNC_HTTP_MAXCONNS` variable, so use that when it is available.

[0]: https://metacpan.org/pod/Net::Async::HTTP#max_connections_per_host-=%3E-INT

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

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

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.

This reverts commit f5ba95e.

Unneeded since we're not using Catalyst::Log here (which doesn't seem to
truly autoflush STDERR by default.)

Dockerize the MetaCPAN