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>

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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.