Push an abridged README to Docker Hub

The full README.md exceeds the character limit for the description on
Docker Hub. This commit adds a shorter READE-containers.md, which is
then used as the container description (on both Docker Hub ond Quay,
for consistency). It contains a reference to the full README.md.

Signed-off-by: beorn7 <[email protected]>

README-containers.md

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+# Prometheus Pushgateway
+
+[![CircleCI](https://circleci.com/gh/prometheus/pushgateway/tree/master.svg?style=shield)][circleci]
+[![Docker Repository on Quay](https://quay.io/repository/prometheus/pushgateway/status)][quay]
+[![Docker Pulls](https://img.shields.io/docker/pulls/prom/pushgateway.svg?maxAge=604800)][hub]
+
+The Prometheus Pushgateway exists to allow ephemeral and batch jobs to
+expose their metrics to Prometheus. Since these kinds of jobs may not
+exist long enough to be scraped, they can instead push their metrics
+to a Pushgateway. The Pushgateway then exposes these metrics to
+Prometheus.
+
+## Non-goals
+
+First of all, the Pushgateway is not capable of turning Prometheus into a
+push-based monitoring system. For a general description of use cases for the
+Pushgateway, please read [When To Use The
+Pushgateway](https://prometheus.io/docs/practices/pushing/).
+
+The Pushgateway is explicitly not an _aggregator or distributed counter_ but
+rather a metrics cache. It does not have
+[statsd](https://github.com/etsy/statsd)-like semantics. The metrics pushed are
+exactly the same as you would present for scraping in a permanently running
+program. If you need distributed counting, you could either use the actual
+statsd in combination with the [Prometheus statsd
+exporter](https://github.com/prometheus/statsd_exporter), or have a look at the
+[prom-aggregation-gateway](https://github.com/zapier/prom-aggregation-gateway). With more
+experience gathered, the Prometheus project might one day be able to provide a
+native solution, separate from or possibly even as part of the Pushgateway.
+
+For machine-level metrics, the
+[textfile](https://github.com/prometheus/node_exporter/blob/master/README.md#textfile-collector)
+collector of the Node exporter is usually more appropriate. The Pushgateway is
+intended for service-level metrics.
+
+The Pushgateway is not an _event store_. While you can use Prometheus as a data
+source for
+[Grafana annotations](http://docs.grafana.org/reference/annotations/), tracking
+something like release events has to happen with some event-logging framework.
+
+A while ago, we
+[decided to not implement a “timeout” or TTL for pushed metrics](https://github.com/prometheus/pushgateway/issues/19)
+because almost all proposed use cases turned out to be anti-patterns we
+strongly discourage. You can follow a more recent discussion on the
+[prometheus-developers mailing list](https://groups.google.com/forum/#!topic/prometheus-developers/9IyUxRvhY7w).
+
+## Run it
+
+Download binary releases for your platform from the
+[release page](https://github.com/prometheus/pushgateway/releases) and unpack
+the tarball.
+
+If you want to compile yourself from the sources, you need a working Go
+setup. Then use the provided Makefile (type `make`).
+
+For the most basic setup, just start the binary. To change the address
+to listen on, use the `--web.listen-address` flag (e.g. "0.0.0.0:9091" or ":9091").
+By default, Pushgateway does not persist metrics. However, the `--persistence.file` flag
+allows you to specify a file in which the pushed metrics will be
+persisted (so that they survive restarts of the Pushgateway).
+
+### Using Docker
+
+You can deploy the Pushgateway using the [prom/pushgateway](https://hub.docker.com/r/prom/pushgateway) Docker image.
+
+For example:
+
+```bash
+docker pull prom/pushgateway
+
+docker run -d -p 9091:9091 prom/pushgateway
+```
+
+## More documentation
+
+Please refer to the [full length
+README.md](https://github.com/prometheus/pushgateway/blob/master/README.md) for
+more documentation.