What does this PR do?

It explicitly uses the "Docker" provider at CI time, adding unit tests for the New provider method.

Why is it important?

We discovered that elastic-package tool is having some issues after merging the kibana custom configurations, causing the stack to simply not boot up:

ERRO[2021-10-28T10:06:03+02:00] Error executing command args="[run github.com/elastic/elastic-package stack up --daemon --verbose --version 8.0.0-0da7fd6d-SNAPSHOT --services elasticsearch,fleet-server,kibana,package-registry -p default]" baseDir=. command=go env="map[kibanaDockerNamespace:kibana kibanaProfile:default kibanaVersion:8.0.0-0da7fd6d-SNAPSHOT stackPlatform:linux/amd64 stackVersion:8.0.0-0da7fd6d-SNAPSHOT]" error="exit status 1" stderr="2021/10/28 10:05:46 WARN CommitHash is undefined, in both /Users/mdelapenya/.elastic-package/version and the compiled binary, config may be out of date.\n2021/10/28 10:05:46 DEBUG Enable verbose logging\n2021/10/28 10:05:46 DEBUG Distribution built without a version tag, can't determine release chronology. Please consider using official releases at https://github.com/elastic/elastic-package/releases\nBoot up the Elastic stack\nUsing profile /Users/mdelapenya/.elastic-package/profiles/default.\nRemember to load stack environment variables using 'eval "$(elastic-package stack shellinit)"'.\n2021/10/28 10:05:46 DEBUG running command: /usr/local/bin/docker-compose -f /Users/mdelapenya/.elastic-package/profiles/default/stack/snapshot.yml -p elastic-package-stack build elasticsearch elasticsearch_is_ready fleet-server fleet-server_is_ready kibana kibana_is_ready package-registry package-registry_is_ready\nelasticsearch uses an image, skipping\nelasticsearch_is_ready uses an image, skipping\nkibana uses an image, skipping\nkibana_is_ready uses an image, skipping\npackage-registry_is_ready uses an image, skipping\nfleet-server uses an image, skipping\nfleet-server_is_ready uses an image, skipping\nBuilding package-registry\n#1 [internal] load build definition from Dockerfile.package-registry\n#1 sha256:8fd7d6b6f7573458b9c4d193f0e18737a0940a1fe55ca1cca2c59f10f7a9243c\n#1 transferring dockerfile: 270B done\n#1 DONE 0.0s\n\n#2 [internal] load .dockerignore\n#2 sha256:a884d5f4b9a929bfc82af70bb8b85e822baf8d61bf55afaec10048c185974f3d\n#2 transferring context: 2B done\n#2 DONE 0.0s\n\n#3 [internal] load metadata for docker.elastic.co/package-registry/distribution:snapshot\n#3 sha256:6af08ea3fe5660bb1b6f5eb063bda9105cd11529191487d2f667c65bfc352ebb\n#3 DONE 0.0s\n\n#4 [1/3] FROM docker.elastic.co/package-registry/distribution:snapshot\n#4 sha256:dca3dcd2a75adb4017f6282e95e97cb50e95385327b7cfe60a08ff342f2432fb\n#4 DONE 0.0s\n\n#5 [internal] load build context\n#5 sha256:c491da08728c45155a4987621eef8f2224292961f04cb1097dc9d6253f879826\n#5 transferring context: 360B done\n#5 DONE 0.0s\n\n#6 [2/3] COPY profiles/default/stack/package-registry.config.yml /package-registry/config.yml\n#6 sha256:e6f1e8318df405582c17a84bb3e95803c980451c3e89ede6c1ab83b39931280d\n#6 CACHED\n\n#7 [3/3] COPY stack/development/ /packages/development\n#7 sha256:e06545c901272c50575b09342f277af61a2b3a6ff094af11f20b00456534eac9\n#7 CACHED\n\n#8 exporting to image\n#8 sha256:e8c613e07b0b7ff33893b694f7759a10d42e180f2b4dc349fb57dc6b71dcab00\n#8 exporting layers done\n#8 writing image sha256:c768084dd6e15efafd266de6dca328b6273496d8511a1708a66103bc99b2c1a0 done\n#8 naming to docker.io/library/elastic-package-stack_package-registry done\n#8 DONE 0.0s\n\nUse 'docker scan' to run Snyk tests against images to find vulnerabilities and learn how to fix them\n2021/10/28 10:05:48 DEBUG running command: /usr/local/bin/docker-compose -f /Users/mdelapenya/.elastic-package/profiles/default/stack/snapshot.yml -p elastic-package-stack up -d elasticsearch elasticsearch_is_ready fleet-server fleet-server_is_ready kibana kibana_is_ready package-registry package-registry_is_ready\nCreating network "elastic-package-stack_default" with the default driver\nCreating elastic-package-stack_package-registry_1 ... \r\nCreating elastic-package-stack_elasticsearch_1 ... \r\nCreating elastic-package-stack_package-registry_1 ... done\r\nCreating elastic-package-stack_elasticsearch_1 ... done\r\nCreating elastic-package-stack_package-registry_is_ready_1 ... \r\nCreating elastic-package-stack_package-registry_is_ready_1 ... done\r\nCreating elastic-package-stack_elasticsearch_is_ready_1 ... \r\nCreating elastic-package-stack_elasticsearch_is_ready_1 ... done\r\n\nERROR: for kibana Container "e254f3653164" is unhealthy.\nEncounte

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have run the Unit tests (make unit-test), and they are passing locally
• I have run the End-2-End tests for the suite I'm working on, and they are passing locally
• I have noticed new Go dependencies (run make notice in the proper directory)

Follow-ups

We will eventually restore the elastic-package provider, once the issues with the tool related to maintenance branches are resolved.