Pin the Redis version to 3.1.2

[hoitomt-verodin]

An npm/node error is being thrown on startup which prevents the webui containier from starting in docker-compose; Step 46 in the training

```
webui_1   | /node_modules/redis/dist/index.js:27
webui_1   |     ...bloom_1.default,
webui_1   |     ^^^
webui_1   |
webui_1   | SyntaxError: Unexpected token ...
webui_1   |     at exports.runInThisContext (vm.js:53:16)
webui_1   |     at Module._compile (module.js:373:25)
webui_1   |     at Object.Module._extensions..js (module.js:416:10)
webui_1   |     at Module.load (module.js:343:32)
webui_1   |     at Function.Module._load (module.js:300:12)
webui_1   |     at Module.require (module.js:353:17)
webui_1   |     at require (internal/module.js:12:17)
webui_1   |     at Object.<anonymous> (/webui.js:3:13)
webui_1   |     at Module._compile (module.js:409:26)
webui_1   |     at Object.Module._extensions..js (module.js:416:10)
dockercoins_webui_1 exited with code 1
```