BROKEN attempt to demo registrar communication with the blockchain

This is a really weird bug. After running `scripts/run_collector.sh`,
which brings everything up, it's perfectly possible to get into
a state very much like what the registrar is in, and communicate
with the blockchain without issue:

```sh
$ docker run --rm --net cumulus_testing_net para-reg:latest polkadot-js-api --ws ws://172.28.1.1:9944 query.sudo.key
Thu 20 Feb 2020 12:19:20 PM CET
{
  "key": "5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY"
}
```

However, the registrar itself, doing the same thing from within
`register_para.sh`, is failing to find the right place in the network:

```
/runtime/cumulus_test_parachain_runtime.compact.wasm found after 0 seconds
/genesis/genesis-state found after 0 seconds
2020-02-20 10:43:22          API-WS: disconnected from ws://172.28.1.1:9944 code: '1006' reason: 'connection failed'
_Event {
  type: 'error',
  isTrusted: false,
  _yaeti: true,
  target: W3CWebSocket {
    _listeners: {},
    addEventListener: [Function: _addEventListener],
    removeEventListener: [Function: _removeEventListener],
    dispatchEvent: [Function: _dispatchEvent],
    _url: 'ws://172.28.1.1:9944',
    _readyState: 3,
    _protocol: undefined,
    _extensions: '',
    _bufferedAmount: 0,
    _binaryType: 'arraybuffer',
    _connection: undefined,
    _client: WebSocketClient {
      _events: [Object: null prototype] {},
      _eventsCount: 0,
      _maxListeners: undefined,
      config: [Object],
      _req: null,
      protocols: [],
      origin: undefined,
      url: [Url],
      secure: false,
      base64nonce: 'aJ6J3pYDz8l5owVWHGbzHg==',
      [Symbol(kCapture)]: false
    },
    onclose: [Function (anonymous)],
    onerror: [Function (anonymous)],
    onmessage: [Function (anonymous)],
    onopen: [Function (anonymous)]
  },
  cancelable: true,
  stopImmediatePropagation: [Function (anonymous)]
}
```

They should be connected to the same network, running the same
image, doing the same call. The only difference is the file
existence checks, which really shouldn't be affecting the network
state at all.

Pushing this commit to ask for outside opinions on it, because
this is very weird and I clearly don't understand some part of
what's happening.

docker/docker-compose.yml

@@ -106,6 +106,7 @@ services:
       - "genesis-state:/genesis"
       - "parachain-runtime:/runtime"
     depends_on:
+      - node_alice
       - runtime
       - genesis_state
     networks:

docker/parachain-registrar.dockerfile

@@ -1,6 +1,8 @@
 FROM node:latest AS pjs
 
-RUN yarn global add @polkadot/api-cli
+# It would be great to depend on a more stable tag, but we need some
+# as-yet-unreleased features.
+RUN yarn global add @polkadot/[email protected]
 
 ENTRYPOINT [ "polkadot-js-api" ]
 CMD [ "--version" ]
@@ -15,10 +17,6 @@ CMD [ "--version" ]
 #   pjs query.sudo.key
 
 FROM pjs
-
-# install tools required to run the registration script
-RUN apt-get update && apt-get install -y xxd
-
 # the only thing left to do is to actually run the transaction.
 COPY ./scripts/register_para.sh /usr/bin
 # unset the previous stage's entrypoint

scripts/register_para.sh

@@ -2,14 +2,54 @@
 
 set -e -o pipefail
 
-# this is straightforward: just send the sudo'd tx to the alice node
+sizeof () {
+    stat --printf="%s" "$1"
+}
+
+wait_for_file () {
+    # Wait for a file to have a stable, non-zero size.
+    # Takes at least 0.2 seconds per run, but there's no upper bound if the
+    # file grows continuously. If the file doesn't exist, or stably has 0 size,
+    # this will take up to 10 seconds by default; this limit can be adjusted by
+    # the second input parameter.
+    path="$1"
+    limit="$2"
+    if [ -z "$limit" ]; then
+        limit=10
+    fi
+    count=0
+    while [ "$count" -lt "$limit" ]; do
+        if [ -s "$path" ]; then
+            echo "$path found after $count seconds"
+            # now ensure that the file size is stable: it's not still being written
+            oldsize=0
+            size="$(sizeof "$path")"
+            while [ "$oldsize" -ne "$size" ]; do
+                sleep 0.2
+                oldsize="$size"
+                size="$(sizeof "$path")"
+            done
+            return
+        fi
+        count=$((count+1))
+        sleep 1
+    done
+    echo "$path not found after $count seconds"
+    exit 1
+}
+
+wait_for_file /runtime/cumulus_test_parachain_runtime.compact.wasm
+wait_for_file /genesis/genesis-state
+
+# this is now straightforward: just send the sudo'd tx to the alice node
 
 polkadot-js-api \
     --ws ws://172.28.1.1:9944 \
-    --sudo \
-    --seed "//Alice" \
-    tx.registrar.registerPara \
-        100 \
-        '{"scheduling":"Always"}' \
-        @/runtime/cumulus_test_parachain_runtime.compact.wasm \
-        @/genesis/genesis-state
+    query.sudo.key
+    # --sudo \
+    # --seed "//Alice" \
+    # tx.registrar.registerPara \
+    #     100 \
+    #     '{"scheduling":"Always"}' \
+    #     @/runtime/cumulus_test_parachain_runtime.compact.wasm \
+    #     @/genesis/genesis-state