Dockerfile: bump gh-pages to v177

This is needed for the following reasons:

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

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Archive for v17.09

Improve archival dockerfile (#5355)

Improve archival dockerfile (#5351)

Improve archival dockerfile (#5358)

Improve archival dockerfile (#5359)

Improve archival dockerfile (#5360)

Improve archival dockerfile (#5361)

Improve archival dockerfile (#5364)

Improve archival dockerfile (#5365)