docker-compose: fix the assets container

[decentral1se]

When bringing up the dev site, everything is broken :) Fix incoming ...

[decentral1se]

Righto, the gulp machinery was dropped (see https://github.com/cotech/website/pull/90/files#r167916834).

Following that, I found the next hurdle:

assets_1      | npm WARN optional SKIPPING OPTIONAL DEPENDENCY: opts.mkdirpAsync is not a function
assets_1      | npm WARN optional SKIPPING OPTIONAL DEPENDENCY: ansi-wrap@0.1.0 (node_modules/ansi-wrap):
assets_1      | npm WARN optional SKIPPING OPTIONAL DEPENDENCY: opts.mkdirpAsync is not a function
assets_1      | npm WARN optional SKIPPING OPTIONAL DEPENDENCY: ansi-styles@2.2.1 (node_modules/ansi-styles):
assets_1      | npm WARN optional SKIPPING OPTIONAL DEPENDENCY: opts.mkdirpAsync is not a function
assets_1      | npm WARN optional SKIPPING OPTIONAL DEPENDENCY: ansi-regex@2.1.1 (node_modules/ansi-regex):
assets_1      | npm WARN optional SKIPPING OPTIONAL DEPENDENCY: opts.mkdirpAsync is not a function
assets_1      | npm WARN optional SKIPPING OPTIONAL DEPENDENCY: ansi-gray@0.1.1 (node_modules/ansi-gray):
assets_1      | npm WARN optional SKIPPING OPTIONAL DEPENDENCY: opts.mkdirpAsync is not a function
assets_1      | 
assets_1      | npm ERR! opts.mkdirpAsync is not a function

So, it seems our installed npm version is broken in some way. I've upgrade that.