Refactor docker locations and README.

This addresses #3224

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+# Synapse Docker
+
+This Docker image will run Synapse as a single process. It does not provide a database
+server or a TURN server, you should run these separately.
+
+## Run
+
+We do not currently offer a `latest` image, as this has somewhat undefined semantics.
+We instead release only tagged versions so upgrading between releases is entirely
+within your control.
+
+### Using docker-compose (easier)
+
+This image is designed to run either with an automatically generated configuration
+file or with a custom configuration that requires manual edition.
+
+An easy way to make use of this image is via docker-compose, see the (https://github.com/matrix-org/synapse/tree/develop/contrib/docker)[contrib] section of the synapse project for examples.
+
+### Without Compose (harder)
+
+If you do not wish to use Compose, you may still run this image using plain
+Docker commands. Note that the following is just a guideline and you may need
+to add parameters to the docker run command to account for the network situation
+with your postgres database.
+
+```
+docker run \
+    -d \
+    --name synapse \
+    -v ${DATA_PATH}:/data \
+    -e SYNAPSE_SERVER_NAME=my.matrix.host \
+    -e SYNAPSE_REPORT_STATS=yes \
+    docker.io/matrixdotorg/synapse:latest
+```
+
+## Volumes
+
+The image expects a single volume, located at ``/data``, that will hold:
+
+* temporary files during uploads;
+* uploaded media and thumbnails;
+* the SQLite database if you do not configure postgres;
+* the appservices configuration.
+
+You are free to use separate volumes depending on storage endpoints at your
+disposal. For instance, ``/data/media`` coud be stored on a large but low
+performance hdd storage while other files could be stored on high performance
+endpoints.
+
+In order to setup an application service, simply create an ``appservices``
+directory in the data volume and write the application service Yaml
+configuration file there. Multiple application services are supported.
+
+## Environment
+
+Unless you specify a custom path for the configuration file, a very generic
+file will be generated, based on the following environment settings.
+These are a good starting point for setting up your own deployment.
+
+Global settings:
+
+* ``UID``, the user id Synapse will run as [default 991]
+* ``GID``, the group id Synapse will run as [default 991]
+* ``SYNAPSE_CONFIG_PATH``, path to a custom config file
+
+If ``SYNAPSE_CONFIG_PATH`` is set, you should generate a configuration file
+then customize it manually. No other environment variable is required.
+
+Otherwise, a dynamic configuration file will be used. The following environment
+variables are available for configuration:
+
+* ``SYNAPSE_SERVER_NAME`` (mandatory), the current server public hostname.
+* ``SYNAPSE_REPORT_STATS``, (mandatory, ``yes`` or ``no``), enable anonymous
+  statistics reporting back to the Matrix project which helps us to get funding.
+* ``SYNAPSE_NO_TLS``, set this variable to disable TLS in Synapse (use this if
+  you run your own TLS-capable reverse proxy).
+* ``SYNAPSE_ENABLE_REGISTRATION``, set this variable to enable registration on
+  the Synapse instance.
+* ``SYNAPSE_ALLOW_GUEST``, set this variable to allow guest joining this server.
+* ``SYNAPSE_EVENT_CACHE_SIZE``, the event cache size [default `10K`].
+* ``SYNAPSE_CACHE_FACTOR``, the cache factor [default `0.5`].
+* ``SYNAPSE_RECAPTCHA_PUBLIC_KEY``, set this variable to the recaptcha public
+  key in order to enable recaptcha upon registration.
+* ``SYNAPSE_RECAPTCHA_PRIVATE_KEY``, set this variable to the recaptcha private
+  key in order to enable recaptcha upon registration.
+* ``SYNAPSE_TURN_URIS``, set this variable to the coma-separated list of TURN
+  uris to enable TURN for this homeserver.
+* ``SYNAPSE_TURN_SECRET``, set this to the TURN shared secret if required.
+
+Shared secrets, that will be initialized to random values if not set:
+
+* ``SYNAPSE_REGISTRATION_SHARED_SECRET``, secret for registrering users if
+  registration is disable.
+* ``SYNAPSE_MACAROON_SECRET_KEY`` secret for signing access tokens
+  to the server.
+
+Database specific values (will use SQLite if not set):
+
+* `POSTGRES_DB` - The database name for the synapse postgres database. [default: `synapse`]
+* `POSTGRES_HOST` - The host of the postgres database if you wish to use postgresql instead of sqlite3. [default: `db` which is useful when using a container on the same docker network in a compose file where the postgres service is called `db`]
+* `POSTGRES_PASSWORD` - The password for the synapse postgres database. **If this is set then postgres will be used instead of sqlite3.** [default: none] **NOTE**: You are highly encouraged to use postgresql! Please use the compose file to make it easier to deploy.
+* `POSTGRES_USER` - The user for the synapse postgres database. [default: `matrix`]
+
+Mail server specific values (will not send emails if not set):
+
+* ``SYNAPSE_SMTP_HOST``, hostname to the mail server.
+* ``SYNAPSE_SMTP_PORT``, TCP port for accessing the mail server [default ``25``].
+* ``SYNAPSE_SMTP_USER``, username for authenticating against the mail server if any.
+* ``SYNAPSE_SMTP_PASSWORD``, password for authenticating against the mail server if any.
+
+## Build
+
+Build the docker image with the `docker build` command from the root of the synapse repository.
+
+```
+docker build -t docker.io/matrixdotorg/synapse . -f docker/Dockerfile
+```
+
+The `-t` option sets the image tag. Official images are tagged `matrixdotorg/synapse:<version>` where `<version>` is the same as the release tag in the synapse git repository.
+
+You may have a local Python wheel cache available, in which case copy the relevant
+packages in the ``cache/`` directory at the root of the project.