[Agent] Introduce mage demo command

[michalpristas]

What does this PR do?

Introducing mage demo command, which runs a current agent in a container.
command first checks whether image with name elastic-agent-{commit} already exists and if not it prepares a build and create an image, then it runs the container
If docker image already exists it runs the image straight away.

Command needs to be run from within x-pack/agent directory

By default it will enroll to local kibana at locahost:5601. It will use default credentials.
To tweak this behavior use these env variables which will be passed to docker image (default values in square brackets):

FLEET_SETUP - if set to 1 fleet setup will be performed creating a fleet user [false]
FLEET_ADMIN_PASSWORD - used for new fleet user [elastic]
FLEET_ADMIN_USERNAME - used for new fleet user [changeme]
FLEET_CONFIG_ID - config related to new token [defaul]
FLEET_ENROLLMENT_TOKEN - existing enrollment token to be used for enroll
FLEET_TOKEN_NAME - token name for a token to be created
KIBANA_HOST - actual kibana host [http://localhost:5601]
KIBANA_PASSWORD - password for accessing kibana API [changeme]
KIBANA_USERNAME - username for accessing kibana API [elastic]

Why is it important?

To helps devs running an agent without need of tweaking anything.

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 added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

Pinging @elastic/ingest-management (Project:fleet)

[ph]

@michalpristas I've been trying to test this out locally without luck, I've started with a cleaned environment and I only get an issue that it cannot pull the elastic agent. I've run the following commands:

• docker system prune -a
• git reset --hard HEAD
• git clean -fxd
• mage demo

The result I get is:

>> Building using: cmd='build/mage-linux-amd64 golangCrossBuild', env=[CC=o64-clang, CXX=o64-clang++, GOARCH=amd64, GOARM=, GOOS=darwin, PLATFORM_ID=darwin-amd64]
>> Building using: cmd='build/mage-linux-amd64 buildGoDaemon', env=[CC=gcc, CXX=g++, GOARCH=amd64, GOARM=, GOOS=linux, PLATFORM_ID=linux-amd64]
>> Building using: cmd='build/mage-linux-amd64 golangCrossBuild', env=[CC=gcc, CXX=g++, GOARCH=386, GOARM=, GOOS=linux, PLATFORM_ID=linux-386]
/tmp/ccCnN3gJ.o: In function `main':
god.c:(.text+0x2e0): warning: Using 'getgrnam' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
god.c:(.text+0x279): warning: Using 'getpwnam' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
/tmp/ccOSMPL0.o: In function `main':
god.c:(.text+0x2a6): warning: Using 'getgrnam' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
god.c:(.text+0x23e): warning: Using 'getpwnam' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
>> package: Building elastic-agent type=docker for platform=linux/amd64
>> package: Building elastic-agent type=tar.gz for platform=linux/amd64
>> package: Building elastic-agent type=tar.gz for platform=linux/386
Unable to find image 'elastic-agent:latest' locally
docker: Error response from daemon: pull access denied for elastic-agent, repository does not exist or may require 'docker login': denied: requested access to the resource is denied.
See 'docker run --help'.
Error: running "docker run --network host -e FLEET_ENROLL=1 elastic-agent" failed with exit code 125

[ph]

I also did a double check, I do not have anything set or customized in my docker environment, I am running it with the default options.

[ph]

LGTM, not sure over the weekend what fixed my dev env.. :/