chore: replace deprecated function usages (`filepath.HasPrefix` and `client.IsErrNotFound`)

Describe the bug

The codebase contains a few function calls that have been deprecated by their respective upstream libraries. Continuing to use them creates technical debt and makes future library upgrades difficult.

• pkg/connectors/container_client.go:232: Uses client.IsErrNotFound(err) from the Docker SDK, which is deprecated.
• cmd/importDir_test.go:71: Uses filepath.HasPrefix(path, root), which has been deprecated since Go 1.0 because it does not respect path boundaries.

Expected behavior

The codebase should use modern, supported equivalents for these operations.

Actual behavior

Deprecated functions are used, triggering linter warnings (SA1019).

How to Reproduce?

Reproducer:

1. Run staticcheck ./... locally.
2. Observe the deprecation warnings (SA1019) for both functions.

Microcks version or git rev

main (Latest)

Install method (docker-compose, helm chart, operator, docker-desktop extension,...)

CLI

Additional information

This is a good first issue.

• client.IsErrNotFound(err) should be replaced with cerrdefs.IsNotFound(err).
• filepath.HasPrefix should be replaced with strings.HasPrefix(path, root+string(filepath.Separator)).

Start with pkg/connectors/container_client.go:232 and cmd/importDir_test.go:71, then run staticcheck ./... to reproduce the SA1019 warnings. Replace the deprecated calls with the equivalents named in the issue and run the relevant tests plus staticcheck; done means both warnings are gone.