mz868: tolerate dangling symlink when resolving COPY destination

[mzihlmann]

Fixes #868

A COPY whose destination resolves through a dangling symlink aborted the build. resolveIfSymlink walks the destination and calls filepath.EvalSymlinks on the deepest existing component, which lstats the missing target of a dangling link and returns an error, surfaced as failed to eval symlinks. docker writes through the link instead and builds fine. This drives the walk off EvalSymlinks directly and treats its IsNotExist result, whether the path is missing or points through a dangling symlink, as a component to be created rather than a hard error, so the destination resolves to the lexical path and the copy proceeds.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/commands/copy.go	87.50%	0 Missing and 1 partial ⚠️

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