Configure nginx to redirect to 404.html

Change keyserver for nginx

Use shallow clone and shallow checkout

We are compliant with it; we need to fork the source repo that generates this so we can iterate when we please, however.

1. It does not make sense to run apk many times.

2. Use --no-cache flag to avoid doing update, as per
   https://github.com/gliderlabs/docker-alpine/blob/master/docs/usage.md#disabling-cache

Signed-off-by: Kir Kolyshkin <[email protected]>

This is needed for the following reasons:

1. gh-pages >= 174 is required to fix the issue docker#4178 [1]

2. Not using a tag leads to pulling some unknown version
of the image, which might or might not work.

3. Not using a tag means the image is never rebuilt.

NOTE: it was not possible to use a tag before, as recent gh-pages
releases were not tagged on dockerhub; this is now fixed
(see Starefossen/docker-github-pages#36).

[1] docker#4178

Signed-off-by: Kir Kolyshkin <[email protected]>