docs: add chapter to test TBC scripts in containers

This project centralizes, tests, incubates some `to-be-continuous` common stuff (mainly scripts).

## Test TBC scripts in containers

1. run the container (alpine, debian, ~~fedora~~)

    ```bash

    docker run --rm -it --entrypoint /bin/sh --volume $(pwd):/app --workdir /app alpine

    ```

2. in the container

    ```bash

    export XDG_CACHE_HOME=/app/.cache

    set +e

    # test maybe_install_packages

    source tbc_base.sh

    source tbc_maybe_install_packages.sh

    maybe_install_packages wget ca-certificates git

    # test unscope_variables

    source tbc_base.sh

    source tbc_unscope_vars.sh

    export CI_JOB_STAGE=test

    export scoped__TESTVAR1__if__CI_JOB_STAGE__equals__test="stage is 'test'"

    export scoped__TESTVAR2__if__CI_JOB_STAGE__equals__abcde="stage is 'abcde'"

    export scoped__TESTVAR3__if__CI_JOB_STAGE__in__build__test="stage is 'build' or 'test'"

    export scoped__TESTVAR4__if__CI_JOB_STAGE__in__package__package_test="stage is 'package' or 'package-test'"

    export scoped__TESTVAR5__if__CI_JOB_STAGE__startswith__te="stage starts with 'te'" # problème sur Alpine - non sélectionné

    export scoped__TESTVAR6__if__CI_JOB_STAGE__startswith__ab="stage starts with 'ab'"

    export scoped__TESTVAR7__if__CI_JOB_STAGE__endswith__st="stage ends with 'st'"

    export scoped__TESTVAR8__if__CI_JOB_STAGE__endswith__de="stage ends with 'de'"

    export scoped__TESTVAR9__if__CI_JOB_STAGE__contains__es="stage contains 'es'" # problème sur Alpine - tests: unknown operand

    export scoped__TESTVAR10__if__CI_JOB_STAGE__contains__bcd="stage contains 'bcd'"

    unscope_variables

    ...

    ```