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/alpine/users.go

@@ -1,7 +1,6 @@
 package alpine
 
 import (
-	"errors"
 	"fmt"
 	"io/fs"
 	"os"
@@ -9,8 +8,6 @@ import (
 	"strconv"
 )
 
-var ErrUserAlreadyExists = errors.New("user already exists")
-
 // CreateUser creates a user in Alpine with the given UID.
 func (a *Alpine) CreateUser(username string, uid int) (createdUsername string, err error) {
 	UIDStr := strconv.Itoa(uid)
@@ -34,8 +31,8 @@ func (a *Alpine) CreateUser(username string, uid int) (createdUsername string, e
 	}
 
 	if u != nil {
-		return "", fmt.Errorf("%w: with name %s for ID %s instead of %d",
-			ErrUserAlreadyExists, username, u.Uid, uid)
+		return "", fmt.Errorf("user already exists: with name %s for ID %s instead of %d",
+			username, u.Uid, uid)
 	}
 
 	const permission = fs.FileMode(0o644)