docs: document monorepo and polirepo container pipelines

grype:

  rules:

    - when: never

##### USAGE #####

# TODO: move to readme?

# by default, no job will run.

# the user needs to define manually a kaniko job and a grype job for each of his container images

# see tests/pipelines/container/monorepo.yml for an example

# container jobs

We support 2 type of projects:

* polirepos: projects with only one container image

* monorepos: projects with many container images

## polirepos

Including the following you will automatically get a `kaniko` job and a `grype` job to test the image build by `kaniko`.

See `docker/kaniko.yml` for the environment variables available to customise `Dockerfile` location, build context, arguments and more.

```bash

include:

    - project: 'just-ci/templates'

      file: 'pipelines/container/polirepo.yml'

```

## monorepos

The user must include the following

```bash

include:

    - project: 'just-ci/templates'

      file: 'pipelines/container/monorepo.yml'

```

However, by default no job will run. The user needs to define manually a kaniko job and a grype job for each of his container images. This is because we can not infer the location of the container contexts. For example a user with 2 container images in directories `image_A` and `image_B` in his project will need to add the following:

```yaml

# see tests/pipelines/container/monorepo.yml for a real example

kaniko:image_A:

  extends: .kaniko:monorepo:

  variables:

    CONTEXT_PATH: image_A

kaniko:image_B:

  extends: .kaniko:monorepo:

  variables:

    CONTEXT_PATH: image_B

grype:image_A:

  extends: .grype:monorepo

  variables:

    GRYPE_CONTEXT: image_A

  needs: ["kaniko:image_A"]

grype:image_B:

  extends: .grype:monorepo

  variables:

    GRYPE_CONTEXT: image_B

  needs: ["kaniko:image_B"]

```