Document the need to use a PAT with Registry when 2FA is on

GitLab 8.12 introduced a new permissions model which tightened the
security of Container Registry. It is now required to use a personal
token if 2FA is enabled.

[ci skip]

doc/ci/docker/using_docker_build.md

@@ -270,12 +270,16 @@ which can be avoided if a different driver is used, for example `overlay`.
 ## Using the GitLab Container Registry
-> **Note:**
-This feature requires GitLab 8.8 and GitLab Runner 1.2.
-
-Once you've built a Docker image, you can push it up to the built-in [GitLab Container Registry](../../user/project/container_registry.md). For example, if you're using
-docker-in-docker on your runners, this is how your `.gitlab-ci.yml` could look:
+> **Notes:**
+- This feature requires GitLab 8.8 and GitLab Runner 1.2.
+- Starting from GitLab 8.12, if you have 2FA enabled in your account, you need
+  to pass a personal access token instead of your password in order to login to
+  GitLab's Container Registry.
+Once you've built a Docker image, you can push it up to the built-in
+[GitLab Container Registry](../../user/project/container_registry.md). For example,
+if you're using docker-in-docker on your runners, this is how your `.gitlab-ci.yml`
+could look like:
 ```yaml
  build:
@@ -354,10 +358,20 @@ deploy:
 ```
 Some things you should be aware of when using the Container Registry:
-* You must log in to the container registry before running commands. Putting this in `before_script` will run it before each build job.
-* Using `docker build --pull` makes sure that Docker fetches any changes to base images before building just in case your cache is stale. It takes slightly longer, but means you don’t get stuck without security patches to base images.
-* Doing an explicit `docker pull` before each `docker run` makes sure to fetch the latest image that was just built. This is especially important if you are using multiple runners that cache images locally. Using the git SHA in your image tag makes this less necessary since each build will be unique and you shouldn't ever have a stale image, but it's still possible if you re-build a given commit after a dependency has changed.
-* You don't want to build directly to `latest` in case there are multiple builds happening simultaneously.
+
+- You must log in to the container registry before running commands. Putting
+  this in `before_script` will run it before each build job.
+- Using `docker build --pull` makes sure that Docker fetches any changes to base
+  images before building just in case your cache is stale. It takes slightly
+  longer, but means you don’t get stuck without security patches to base images.
+- Doing an explicit `docker pull` before each `docker run` makes sure to fetch
+  the latest image that was just built. This is especially important if you are
+  using multiple runners that cache images locally. Using the git SHA in your
+  image tag makes this less necessary since each build will be unique and you
+  shouldn't ever have a stale image, but it's still possible if you re-build a
+  given commit after a dependency has changed.
+- You don't want to build directly to `latest` in case there are multiple builds
+  happening simultaneously.
 [docker-in-docker]: https://blog.docker.com/2013/09/docker-can-now-run-within-docker/
 [docker-cap]: https://docs.docker.com/engine/reference/run/#runtime-privilege-and-linux-capabilities

doc/user/project/container_registry.md

@@ -4,13 +4,15 @@
 ---
-> **Note**
-Docker Registry manifest `v1` support was added in GitLab 8.9 to support Docker
-versions earlier than 1.10.
->
-This document is about the user guide. To learn how to enable GitLab Container
-Registry across your GitLab instance, visit the
-[administrator documentation](../../administration/container_registry.md).
+>**Notes:**
+- Docker Registry manifest `v1` support was added in GitLab 8.9 to support Docker
+  versions earlier than 1.10.
+- This document is about the user guide. To learn how to enable GitLab Container
+  Registry across your GitLab instance, visit the
+  [administrator documentation](../../administration/container_registry.md).
+- Starting from GitLab 8.12, if you have 2FA enabled in your account, you need
+  to pass a personal access token instead of your password in order to login to
+  GitLab's Container Registry.
 With the Docker Container Registry integrated into GitLab, every project can
 have its own space to store its Docker images.

doc/user/project/new_ci_build_permissions_model.md

@@ -187,11 +187,17 @@ To properly configure submodules with GitLab CI, read the
 With the update permission model we also extended the support for accessing
 Container Registries for private projects.
-> **Note:**
-As GitLab Runner 1.6 doesn't yet incorporate the introduced changes for
-permissions, this makes the `image:` directive to not work with private projects
-automatically. The manual configuration by an Administrator is required to use
-private images. We plan to remove that limitation in one of the upcoming releases.
+> **Notes:**
+- GitLab Runner versions prior to 1.8 don't incorporate the introduced changes
+  for permissions. This makes the `image:` directive to not work with private
+  projects automatically and it needs to be configured manually on Runner's host
+  with a predefined account (for example administrator's personal account with
+  access token created explicitly for this purpose). This issue is resolved with
+  latest changes in GitLab Runner 1.8 which receives GitLab credentials with
+  build data.
+- Starting with GitLab 8.12, if you have 2FA enabled in your account, you need
+  to pass a personal access token instead of your password in order to login to
+  GitLab's Container Registry.
 Your builds can access all container images that you would normally have access
 to. The only implication is that you can push to the Container Registry of the