[doc] Update shared-docker.asciidoc to reflect correct base OS

[toby-sutor]

According to https://github.com/elastic/obs-infraobs-team/issues/613, we changed the Docker base images from centos to Ubuntu on all Beats systems.

I just confirmed this by following the steps at https://www.elastic.co/guide/en/beats/metricbeat/current/running-on-docker.html.

$ cat /etc/*release*
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=20.04
DISTRIB_CODENAME=focal
DISTRIB_DESCRIPTION="Ubuntu 20.04.6 LTS"
NAME="Ubuntu"
VERSION="20.04.6 LTS (Focal Fossa)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 20.04.6 LTS"
VERSION_ID="20.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal

As such, we need to update the Beats documentation for all Beats, like Filebeat and Metricbeat, to reflect those changes. This should be backported to all Beats versions up to 8.1.0.

[elasticmachine]

Pinging @elastic/ingest-docs (Team:Docs)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @toby-sutor? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2023-06-26T07:52:18.695+0000

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[pierrehilbert]

Good catch! @kilfoyle

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @toby-sutor? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[kilfoyle]

LGTM! Thanks a lot Toby!

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.