Docker with macOS support

Created a new snowblock that contains the `~/.docker/config.json`
configuration file. The file for the `iceowl` host has been configured
to ensure the macOS Keychain is used to store tokens instead of saving
them as plain text into the config file.

Related to epic GH-131
Closes GH-136

snowblocks/docker/README.md

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+# Docker
+
+> Docker is the leader in the containerization market, an open-source project that automates the deployment of software applications inside containers by providing an additional layer of abstraction and automation of operating-system-level virtualization on Linux.
+
+## Setup
+
+### macOS Keychain
+
+To ensure that registry login tokens are stored in the macOS Keychain instead as plain text in the `~/.docker/config.json` file the configuration file must be adjusted:
+
+```json
+{
+  "credsStore": "osxkeychain"
+}
+```

snowblocks/docker/config.iceowl.json

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+{
+  "credsStore": "osxkeychain"
+}

snowblocks/docker/snowblock.json

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+[
+  {
+    "clean": ["~/.docker"]
+  },
+  {
+    "link": {
+      "~/.docker/config.json": {
+        "create": true,
+        "force": true,
+        "hosts": {
+          "iceowl": "config.iceowl.json"
+        }
+      }
+    }
+  }
+]