Cherry-pick #18408 to 7.8: Fix jq: command not found

[michalpristas]

Cherry-pick of PR #18408 to 7.8 branch. Original message:

What does this PR do?

Fixes jq command not found issue with generated dockerfile by splitting installation in two steps.

Why is it important?

to have jq present so entrypoint script is able to parse fleet responses

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.

How to test it

build Dockerfiles out of templates using

cd x-pack/elastic-agent
PLATFORMS=`+all linux/amd64 mage package

you will find it inside x-pack/elastic-agent/build/package/elastic-agent/elastic-agent-linux-amd64.docker/docker-build

from ^^ directory build docker image and run it

docker build -t agent .
docker run -it --entrypoint /bin/bash agent

verify that jq is present

Fixes: #18406

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Replayed Fails to start with pcap.OpenLive failed(): : No such device exists #2, Started by user Michal Pristas, Rebuilds build The "default config" fallback doesn't work properly #3]

• Start Time: 2020-05-12T05:43:33.390+0000

• Duration: 72 min 38 sec (4297540)

• Commit: c300074

Test stats 🧪

Test	Results
Failed	0
Passed	5886
Skipped	899
Total	6785