Unable to reproduce release build, minified files differ

## Problem

Build pipeline produces different output than one represented by a CID attached to [release notes](https://github.com/ipfs-shipyard/ipfs-webui/releases/tag/v2.3.3), which causes ipfs-companion to be hidden from addons.mozilla.org (https://github.com/ipfs-shipyard/ipfs-companion/issues/679) due to reproducibility issues:

> 2) I've built webui branch https://github.com/ipfs-shipyard/ipfs-webui/releases/tag/v2.3.3 using node 8.12.0 and npm 6.4.1, but the resulting code is different from the webui folder in the xpi. Please see the attached build https://drive.google.com/open?id=1Ba9kvMhSUZM0KMQMhP7ebYmGX8CCAUAh
Please test your build steps on a virtual machine with a clean setup and update the instructions for us.

I compared
- [`build_webui`](https://drive.google.com/open?id=1Ba9kvMhSUZM0KMQMhP7ebYmGX8CCAUAh) provided by reviewer
- and one from `QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ` ([ipfs-webui/releases/tag/v2.3.3](https://github.com/ipfs-shipyard/ipfs-webui/releases/tag/v2.3.3))

Indeed, minified content differs:

[ Click to expand diff ]

```console
$ diff --brief -r build_webui webui_QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ
Files build_webui/asset-manifest.json and webui_QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ/asset-manifest.json differ
Files build_webui/index.html and webui_QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ/index.html differ
Only in build_webui: manifest.json
Only in webui_QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ: _previous-versions
Files build_webui/service-worker.js and webui_QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ/service-worker.js differ
Only in build_webui/static/css: main.ace3c083.css.map
Only in build_webui/static/js: 0.45d2ea26.chunk.js
Only in build_webui/static/js: 0.45d2ea26.chunk.js.map
Only in webui_QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ/static/js: 0.f150ccdd.chunk.js
Only in webui_QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ/static/js: 1.61c650c3.chunk.js
Only in build_webui/static/js: 1.6bbfba19.chunk.js
Only in build_webui/static/js: 1.6bbfba19.chunk.js.map
Only in build_webui/static/js: 2.bc355859.chunk.js
Only in build_webui/static/js: 2.bc355859.chunk.js.map
Only in webui_QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ/static/js: 2.d164b02b.chunk.js
Only in build_webui/static/js: 3.52556504.chunk.js
Only in build_webui/static/js: 3.52556504.chunk.js.map
Only in webui_QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ/static/js: 3.c371693f.chunk.js
Only in webui_QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ/static/js: 4.2f4edf06.chunk.js
Only in build_webui/static/js: 4.cdb20f5c.chunk.js
Only in build_webui/static/js: 4.cdb20f5c.chunk.js.map
Only in webui_QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ/static/js: 5.5fd1145b.chunk.js
Only in build_webui/static/js: 5.e171992c.chunk.js
Only in build_webui/static/js: 5.e171992c.chunk.js.map
Only in build_webui/static/js: 6.6b80439c.chunk.js
Only in build_webui/static/js: 6.6b80439c.chunk.js.map
Only in webui_QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ/static/js: 6.e9a35513.chunk.js
Only in webui_QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ/static/js: 7.9c60492f.chunk.js
Only in build_webui/static/js: 7.ee081174.chunk.js
Only in build_webui/static/js: 7.ee081174.chunk.js.map
Only in build_webui/static/js: main.2b881fc1.js
Only in build_webui/static/js: main.2b881fc1.js.map
Only in webui_QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ/static/js: main.bf82967b.js
```

## Looking for a Solution

So far options are:

- **(A)** Recreate release build and provide step-by-step instruction for reviewer
- Are we able to provide steps on how to reproduce minified output from `QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ` ?
- Who built it? What was the version of node/npm? Will steps work cross-platform?
- **(B)** Update build pipeline to support reproducible build and make a new release
- Is it technically feasible to provide reproducible build of ipfs-webui that works the same across all platforms?
- We provide docker-based build pipeline for ipfs-companion, which is an overkill but works.
- **(C)** Remove bundled Web UI from IPFS Companion and load it from IPFS
- We may do this anyway just to restore ipfs-companion in Firefox addon store (and add bundled Web UI when it is possible to build in reproducible manner)
- **(D)** ?

@olizilla @alanshaw @hugomrdias would love to hear your thoughts on this