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/loopstate/apply.go

@@ -2,15 +2,12 @@ package loopstate
 
 import (
 	"context"
-	"errors"
 	"fmt"
 
 	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/gluetun/internal/models"
 )
 
-var ErrInvalidStatus = errors.New("invalid status")
-
 // ApplyStatus sends signals to the running loop depending on the
 // current status and status requested, such that its next status
 // matches the requested one. It is thread safe and a synchronous call
@@ -73,7 +70,7 @@ func (s *State) ApplyStatus(ctx context.Context, status models.LoopStatus) (
 		return newStatus.String(), nil
 	default:
 		s.statusMu.Unlock()
-		return "", fmt.Errorf("%w: %s: it can only be one of: %s, %s",
-			ErrInvalidStatus, status, constants.Running, constants.Stopped)
+		return "", fmt.Errorf("invalid status: %s: it can only be one of: %s, %s",
+			status, constants.Running, constants.Stopped)
 	}
 }