solver: solve synchronously

This forces Solve() to be synchronous and ensures operations don't get
discarded because of optimizations.

Improves fix for #14

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

dagger/component.go

@@ -57,7 +57,10 @@ func (c *Component) Compute(ctx context.Context, s Solver, out Fillable) (FS, er
 	if err != nil {
 		return fs, err
 	}
-	_, err = fs.ReadDir(ctx, "/")
+
+	// Force a `Solve()` in case it hasn't been called earlier.
+	// If the FS is already solved, this is a noop.
+	_, err = fs.Solve(ctx)
 	return fs, err
 }