add composer docs

composer/README-short.txt

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+Composer is a dependency manager written in and for PHP.

composer/content.md

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+# What is Composer?
+
+Composer is a tool for dependency management in PHP, written in PHP. It allows you to declare the libraries your project depends on and it will manage (install/update) them for you.
+
+You can read more about Composer in our [official documentation](https://getcomposer.org/doc/).
+
+%%LOGO%%
+
+# Using
+
+Run the `composer` image:
+
+```sh
+docker run --rm --interactive --tty \
+    --volume $PWD:/app \
+    composer install
+```
+
+You can mount the Composer home directory from your host inside the Container to share caching and configuration files:
+
+```sh
+docker run --rm --interactive --tty \
+    --volume $PWD:/app \
+    --volume $COMPOSER_HOME:/composer \
+    composer install
+```
+
+By default, Composer runs as root inside the container. This can lead to permission issues on your host filesystem. You can run Composer as your local user:
+
+```sh
+docker run --rm --interactive --tty \
+    --volume $PWD:/app \
+    --user $(id -u):$(id -g) \
+    composer install
+```
+
+When you need to access private repositories, you will either need to share your configured credentials, or mount your `ssh-agent` socket inside the running container:
+
+```sh
+docker run --rm --interactive --tty \
+    --volume $PWD:/app \
+    --volume $SSH_AUTH_SOCK:/ssh-auth.sock \
+    --env SSH_AUTH_SOCK=/ssh-auth.sock \
+    composer install
+```
+
+## Suggestions
+
+### PHP Extensions
+
+We strive to deliver an image that is as lean as possible, aimed at running Composer only.
+
+Sometimes dependencies or Composer [scripts](https://getcomposer.org/doc/articles/scripts.md) require the availability of certain PHP extensions. In this scenario, you have two options:
+
+-	Pass the `--ignore-platform-reqs` and `--no-scripts` flags to `install` or `update`:
+
+	```sh
+	docker run --rm --interactive --tty \
+	    --volume $PWD:/app \
+	    composer install --ignore-platform-reqs --no-scripts
+	```
+
+-	Create your own image (possibly by extending `FROM composer`).
+
+### Local runtime/binary
+
+If you want to be able to run `composer` as if it was installed on your host locally, you can define the following function in your `~/.bashrc`, `~/.zshrc` or similar:
+
+```sh
+composer () {
+    tty=
+    tty -s && tty=--tty
+    docker run \
+        $tty \
+        --interactive \
+        --rm \
+        --user $(id -u):$(id -g) \
+        --volume $(pwd):/app \
+        composer "$@"
+}
+```

composer/github-repo

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+https://github.com/composer/docker

composer/license.md

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+View [license information](https://github.com/composer/composer/blob/master/LICENSE) for the software contained in this image.

composer/user-feedback.md

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+## Issues
+
+If you have any problems with or questions about this image, please contact us through a [GitHub issue](%%GITHUB-REPO%%/issues). If the issue is related to a CVE, please check for [a `cve-tracker` issue on the `official-images` repository first](https://github.com/docker-library/official-images/issues?q=label%3Acve-tracker).
+
+You can also reach us through the `#composer` or `#docker-library` IRC channels on [Freenode](https://freenode.net).
+
+## Contributing
+
+You are invited to contribute new features, fixes, or updates, large or small; we are always thrilled to receive pull requests, and do our best to process them as fast as we can.
+
+Before you start to code, we recommend discussing your plans through a [GitHub issue](%%GITHUB-REPO%%/issues), especially for more ambitious contributions. This gives other contributors a chance to point you in the right direction, give you feedback on your design, and help you find out if someone else is working on the same thing.
+
+## Documentation
+
+Documentation for this image is stored in the [`%%REPO%%/` directory](https://github.com/docker-library/docs/tree/master/%%REPO%%) of the [`docker-library/docs` GitHub repo](https://github.com/docker-library/docs). Be sure to familiarize yourself with the [repository's `README.md` file](https://github.com/docker-library/docs/blob/master/README.md) before attempting a pull request.