what's the double arrow (<<: *django) means in dockerfile?
<< and * are both YAML keys (you can also think of them as operators). And one more key related to your question is &.
In YAML, you can define anchors and later use them. For example,
foo: &myanchor key1: "val1" key2: "val2"bar: *myanchorIn this code snippet, & defines an anchor names it myanchor, and *myanchor references that anchor. Now both foo and bar have the same keys and values.
<< is called the YAML merge key. You can compare it to class inheritance in OOP (not so accurate, but can help you understand). See below example.
foo: &myanchor key1: "val1" key2: "val2"bar: << : *myanchor key2: "val2-new" key3: "val3"In this code snippet, we merge keys and values from foo to bar but override the key2 to a new value. And we add a new key-value pair to bar.
Now bar has the following value: {"bar": {"key1": "val1", "key2": val2-new", "key3": "val3"}}.
Hope that helps.