chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/firewall/iptables/ip6tables.go

@@ -82,13 +82,11 @@ func (c *Config) runIP6tablesInstructionNoSave(ctx context.Context, instruction
 	return nil
 }
 
-var ErrPolicyNotValid = errors.New("policy is not valid")
-
 func (c *Config) SetIPv6AllPolicies(ctx context.Context, policy string) error {
 	switch policy {
 	case "ACCEPT", "DROP":
 	default:
-		return fmt.Errorf("%w: %s", ErrPolicyNotValid, policy)
+		return fmt.Errorf("policy is not valid: %s", policy)
 	}
 	return c.runIP6tablesInstructions(ctx, []string{
 		"--policy INPUT " + policy,