Adds OpenGSN docker setup and readme.

gsn/Dockerfile

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+# Building from the CI image (Circle CI)
+FROM cimg/node:16.20.1
+
+WORKDIR /app
+
+RUN git clone --depth 1 --branch v3.0.0-beta.10 \
+  https://github.com/opengsn/gsn.git \
+  /app
+
+# Build the deps, compile the contracts etc.
+RUN yarn create-all-deps && \
+    yarn && \
+    yarn preprocess
+
+# Build the relay server.
+RUN cd packages/relay && \
+    yarn

gsn/README.md

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+# OpenGSN v3 setup ⛽️
+
+The `/gsn` folder within the tt-white-contracts repo contains:
+
+- `deploy`
+- `relay`
+
+It also contains a `Dockerfile` and `compose.yml` file.
+
+Read on for the description...
+
+---
+
+## Docker files
+
+`Dockerfile` contains the minimal setup to pull and build the OpenGSN repo. More specifically it:
+
+- Pulls all the deps
+- Compiles the contracts
+- Compiles the relay server typescript into the js file ready to run
+
+`compose.yml` is the `docker compose` main file.
+
+Running `docker compose up` will bring up **just** the relay server - see "Replay" section below.
+
+Running `docker run gsn_deploy` will deploy the OpenGSN contracts - see "Deploy" section below.
+
+---
+
+## Deploy - dev only
+
+`deploy/config` holds the config for the deployment of the OpenGSN contracts - specifically:
+- `RelayHub`
+- `StakeManager`
+- `Forwarder`
+- `Penalizer`
+- `RelayRegistrar`
+
+Note: This should only be needed on a local chain... we _should_ be using the deployed contracts that exist on chain already.
+
+See:
+- Mainnet network: https://docs.opengsn.org/networks/polygon/polygon.html
+- Mumbai network: https://docs.opengsn.org/networks/polygon/mumbai.html
+
+---
+
+## Relay
+
+`relay` holds the relay server's config / data.

gsn/compose.yml

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+version: "3.8"
+
+services:
+  gsn_deploy:
+    image: ts-opengsn
+    profiles: ["deploy"]
+    build:
+      context: ./
+    working_dir: /app/packages/deployer
+    command: ["yarn", "deploy", "--network", "dev"]
+    networks:
+      - eth_network
+    volumes:
+      - type: bind
+        source: ./deploy/config/deploy.ts
+        target: /app/packages/deployer/deploy/deploy.ts
+      - type: bind
+        source: ./deploy/config/hardhat.config.ts
+        target: /app/packages/deployer/hardhat.config.ts
+      - type: bind
+        source: ./deploy/config/deployment-config.ts
+        target: /app/packages/deployer/deployments/deployment-config.ts
+        # Deployments held in memory.
+      - type: tmpfs
+        target: /app/packages/deployer/deployments/networks
+
+  gsn_relay:
+    image: ts-opengsn
+    build:
+      context: ./
+    working_dir: /app/packages/relay
+    stop_grace_period: 3s
+    command: [
+      "node",
+      "./dist/runServer.js",
+      "--config",
+      "/app/packages/relay/gsn-relay-config.json"
+    ]
+    networks:
+      - eth_network
+    ports:
+      - "8090:8090"
+    volumes:
+      - type: bind
+        source: ./relay/config/gsn-relay-config.json
+        target: /app/packages/relay/gsn-relay-config.json
+        # Relay data held in memory.
+      - type: bind
+        source: ./relay/data
+        target: /gsn-data
+
+networks:
+  eth_network:
+    name: eth_network
+    external: true