Fix .containerignore patterns with leading/trailing slashes

[Honny1]

I checked this with AI. It seems that localbuild takes a different path in the remote environment, causing ignored files to be copied to the server. This issue was largely resolved on Windows and macOS with the recent localapi improvements. However, pure remote builds were still affected. For example, when ignored files were large and ended up being copied to the remote server (not image).

Fixes: #25458

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Does this PR introduce a user-facing change?

Fixed .containerignore/.dockerignore patterns with leading or trailing slashes being silently ignored during remote builds

[mheon]

LGTM

[TomSweeneyRedHat]

LGTM

[TomSweeneyRedHat]

/lgtm