fix: ensure clean state when packaging artifacts

[kruskall]

Motivation/summary

Go’s build process now automatically embeds VCS information into the
binary. When the repository contains untracked files or the working
tree is dirty, the binary receives a “dirty” flag, which can make it
look tampered, confuse security scanners, and break reproducible
builds.

This PR updates the Dockerfiles to copy the entire repository into the
build image and adds all tracked files to the Docker build context.
By ensuring the same source tree is used both inside and outside the
container, the resulting binary matches the locally‑built version and
the Git state remains clean.

The CI packaging pipeline installs gvm directly in the workspace,
producing a gvm binary that isn’t tracked by Git. Because the file
is untracked, the repository appears dirty, causing Go to embed a
dirty flag in the VCS metadata of the built binary.

This change adds the generated gvm binary to .gitignore,
ensuring the Git tree stays clean and the resulting binary’s VCS
metadata reflects a pristine state.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

• build docker image
• inspect apm-server binary and ensure vcs.modified=false

Related issues

Closes #19144

[github-actions]

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• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[mergify]

This pull request does not have a backport label. Could you fix it @kruskall? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-9./d is the label to automatically backport to the 9./d branch. /d is the digit.
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[elasticmachine]

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