`sentry-docker-compose.yml` initial version created (#410)

* Dockerfile fixed.

* docker: use init.sh to setup rust env

* CMD replaced with ENTRYPOINT in Dockefile

* WIP Sentry/Validator basic setup

* Dockerfile modifier to correspond to master version

* `environment` block added to both sentry and validator

* some environment variables are now used to pass params to containers

polkadot/docker/sentry-docker-compose.yml

+# Docker compose file to simulate a sentry node setup.
+#
+#
+# Setup:
+#
+# Validator A is not supposed to be connected to the public internet. Instead it
+# connects to a sentry node (sentry-a) which connects to the public internet.
+#
+#
+# Usage:
+#
+# 1. Build `target/release/substrate` binary: `cargo build --release`
+#
+# 2. Start networks and containers: `sudo docker-compose -f scripts/sentry-node/docker-compose.yml up`
+#
+# 3. Reach:
+#   - validator-a: localhost:9944
+#   - sentry-a: localhost:9946
+
+version: "3.7"
+services:
+  validator:
+    ports:
+      - "9944:9944"
+#    volumes:
+#      - ../../target/release/substrate:/usr/local/bin/substrate
+    image: parity/polkadot
+    networks:
+      - network-a
+    environment:
+        - VALIDATOR_NODE_KEY
+        - VALIDATOR_BASE_PATH
+        - VALIDATOR_CHAIN
+        - VALIDATOR_PORT
+        - VALIDATOR_NAME
+        - VALIDATOR_RESERVED_NODES
+    command:
+      # Local node id: QmRpheLN4JWdAnY7HGJfWFNbfkQCb6tFf4vvA6hgjMZKrR
+      - "--node-key"
+      - "${VALIDATOR_NODE_KEY:-0000000000000000000000000000000000000000000000000000000000000001}"
+      - "--base-path"
+      - "${VALIDATOR_BASE_PATH:-/tmp/alice}"
+      - "--chain=${VALIDATOR_CHAIN:-local}"
+      - "--port"
+      - "${VALIDATOR_PORT:-30333}"
+      - "--validator"
+      - "--name"
+      - "${VALIDATOR_NANE:-AlicesNode}"
+      - "--reserved-nodes"
+      - "${VALIDATOR_RESERVED_NODES:-/dns4/sentry/tcp/30333/p2p/QmV7EhW6J6KgmNdr558RH1mPx2xGGznW7At4BhXzntRFsi}"
+      # Not only bind to localhost.
+      - "--ws-external"
+      - "--rpc-external"
+      # - "--log"
+      # - "sub-libp2p=trace"
+      # - "--log"
+      # - "afg=trace"
+      - "--no-telemetry"
+      - "--rpc-cors"
+      - "all"
+
+  sentry:
+    image: parity/polkadot
+    ports:
+      - "9946:9944"
+#    volumes:
+#      - ../../target/release/substrate:/usr/local/bin/substrate
+    networks:
+      - network-a
+      - internet
+    environment:
+        - SENTRY_NODE_KEY
+        - SENTRY_BASE_PATH
+        - SENTRY_CHAIN
+        - SENTRY_PORT
+        - SENTRY_NAME
+        - SENTRY_BOOTNODES
+    command:
+      # Local node id: QmV7EhW6J6KgmNdr558RH1mPx2xGGznW7At4BhXzntRFsi
+      - "--node-key"
+      - "${SENTRY_NODE_KEY:-0000000000000000000000000000000000000000000000000000000000000003}"
+      - "--base-path"
+      - "${SENTRY_BASE_PATH:-/tmp/charlies}"
+      - "--chain=${SENTRY_CHAIN:-local}"
+      # Don't configure a key, as sentry is not a validator.
+      - "--port"
+      - "${SENTRY_PORT:-30333}"
+      # sentry-a is not a validator.
+      # - "--validator"
+      - "--name"
+      - "${SENTRY_NAME:-CharliesNode}"
+      - "--bootnodes"
+      - "${SENTRY_BOOTNODES:-/dns4/validator-a/tcp/30333/p2p/QmRpheLN4JWdAnY7HGJfWFNbfkQCb6tFf4vvA6hgjMZKrR}"
+      - "--no-telemetry"
+      - "--rpc-cors"
+      - "all"
+      # Not only bind to localhost.
+      - "--ws-external"
+      - "--rpc-external"
+
+networks:
+  network-a:
+  internet: