readonly property of lightning:input not working

You may use attribute disabled instead of readonly.

Here’s an example with lightning:input [checkbox] and lightning:select:

<div>
    Checkboxes
    <lightning:input type="checkbox" label="Red" name="red" checked="true" disabled="true"/>
    <lightning:input type="checkbox" label="Blue" name="blue" disabled="true"/>
</div>

<br/>

<div>
    <lightning:select name="selectItem" label="Select an item" value="2" disabled="true">
        <option value="">choose one...</option>
        <option value="1">one</option>
        <option value="2">two</option>
    </lightning:select>
</div>

HTML docs say that readonly does not work for checkbox and that select does not have a readonly attribute.

Doc for select:

SELECT … does not have a READONLY attribute. The reason is that
technically lists don’t have values… they have selected
options, which themselves have values. You may wish to see , which disables the list.

Doc for checkbox:

It’s important to understand that READONLY merely prevents the user
from changing the value of the field, not from interacting with the
field. For many types of fields, READONLY is irrelevent because you
don’t normally change the value. In checkboxes, for example, you can
check them on or off (thus setting the CHECKED state) but you don’t
change the value of the field. DISABLED, however, actually prevents
you from using the field. Notice in these examples that you can set
the checkboxes even though they are “read only”

Both suggest using the disabled attribute.