Update docs for testnet3 (#773)

* Start docker docs

* Add progress

* Update docker.md

* Add note about geth syncing

* Fix env to use geth

* Update docs

* Remove sigp goerli node

* Change text about eth1 syncing

* Address comments from @pscott

* Add links to beaconcha.in

* Add instructions for starting an eth1 node

* Fix typo

* Re-organise Syncing log

book/src/become-a-validator-docker.md

@@ -0,0 +1,67 @@
+# Become a Validator: Using Docker
+
+Sigma Prime maintains the
+[sigp/lighthouse-docker](https://github.com/sigp/lighthouse-docker) repository
+which provides an easy way to run Lighthouse without building the Lighthouse
+binary yourself.
+
+> Note: when you're running the Docker Hub image you're relying upon a
+> pre-built binary instead of building from source. If you want the highest
+> assurance you're running the _real_ Lighthouse,
+> [build the docker image yourself](./docker.md) instead. You'll need some
+> experience with docker-compose to integrate your locally built docker image
+> with the docker-compose environment.
+
+### 1. Clone the repository
+
+Once you have docker-compose
+[installed](https://docs.docker.com/compose/install/), clone the
+[sigp/lighthouse-docker](https://github.com/sigp/lighthouse-docker) repository.
+
+```bash
+$ git clone https://github.com/sigp/lighthouse-docker
+$ cd lighthouse-docker
+```
+
+### 2. Configure the docker environment
+
+Then, create a file named `.env` with the following contents (these values are
+documented
+[here](https://github.com/sigp/lighthouse-docker/blob/master/default.env)):
+
+```bash
+DEBUG_LEVEL=info
+START_GETH=true
+START_VALIDATOR=true
+VALIDATOR_COUNT=1
+VOTING_ETH1_NODE=http://geth:8545
+DEPOSIT_VALUE=3200000000
+```
+
+_This `.env` file should live in the `lighthouse-docker` directory alongside the
+`docker-compose.yml` file_.
+
+### 3. Start Lighthouse
+
+Start the docker-compose environment (you may need to use `sudo`):
+
+```bash
+$ docker-compose up
+```
+
+> Note: the docker-compose setup includes a fast-synced geth node. You can
+> expect the `beacon_node` to log some eth1-related errors whilst the geth node
+> boots and becomes synced. This will only happen on the first start of the
+> compose environment or if geth loses sync.
+
+### Installation complete!
+
+In the next step you'll need to locate your `eth1_deposit_data.rlp` file from
+your `.lighthouse/validators` directory.
+
+The `./lighthouse` directory is in the root of the `lighthouse-docker`
+repository. For example, if you ran Step 1 in `/home/karlm/` then you can find
+your validator directory in
+`/home/karlm/lighthouse-docker/.lighthouse/validators/`.
+
+You can now go to [Become a Validator: Step 2](become-a-validator.html#2-submit-your-deposit-to-goerli).