build.Dockerfile: Install ca-certificates

[radeksimko]

It turns out we have two Dockerfiles and the other (actually more important) one which is used for building images which then get published e.g. to https://hub.docker.com/r/hashicorp/terraform is currently hiding under .github hierarchy.

To prevent the same mistake in the future I brought it into the root and renamed so that syntax highlighting works for the file "out of the box container".

Finally I carried over the change already merged as part of #36471

Target Release

1.11.x

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[github-actions]

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