docs: improves

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

```bash

# project structure

polirepo

├── .gitlab-ci.yml

├── Dockerfile

├── README.md

├── deployment.yml

└── docker-compose.yml

```

```yml

include:

    - project: 'just-ci/templates'

      file: 'pipelines/container/polirepo.yml'

The user must include the following

```bash

```yml

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

```bash

# project structure

monorepo

├── .gitlab-ci.yml

├── README.md

├── a

│   ├── Dockerfile

│   └── test_A.txt

└── b

    ├── Dockerfile

    └── test_B.txt

```

```yml

kaniko:image_A:

kaniko:a:

  extends: .kaniko:monorepo:

  variables:

    CONTEXT_PATH: image_A

    KANIKO_CONTEXT: a

kaniko:image_B:

kaniko:b:

  extends: .kaniko:monorepo:

  variables:

    CONTEXT_PATH: image_B

    KANIKO_CONTEXT: b

grype:image_A:

grype:a:

  extends: .grype:monorepo

  variables:

    GRYPE_CONTEXT: image_A

  needs: ["kaniko:image_A"]

    GRYPE_CONTEXT: a

  needs: ["kaniko:a"]

grype:image_B:

grype:b:

  extends: .grype:monorepo

  variables:

    GRYPE_CONTEXT: image_B

  needs: ["kaniko:image_B"]

    GRYPE_CONTEXT: b

  needs: ["kaniko:b"]

```