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Line 844 could not think my_dossier_form is a class, except in the case $callback contains an array like array('my_dossier_form', 'methodName');, or it contains a string like 'my_dossier_form::methodName'.

Line 844 could not think my_dossier_form is a class, except in the case $callback contains an array like array('my_dossier_form', 'mymodule_form');, or it contains a string like 'my_dossier_form::methodName'.

The hook implementation done from a Drupal core module, for example comment_forms(), helps to understand the correct code.

The hook implementation done from a Drupal core module, for example comment_forms(), helps to understand the correct code.

As side note, the hook_forms() purpose is providing the same form builder for a group of forms whose ID follow a schema, such as in the case of comment edit forms, whose IDs are comment_node_[node type]_form. When provided, the form builder callback (passed as callback value) should build the full form, not part of it. The fact my_dossier_form() builds just a submission button, and my_dossier_forms() is using the same code used by node_forms() makes me think the code purpose is altering the node edit forms, which should be accomplished by implementing hook_form_BASE_FORM_ID_alter(). That's what the Book module does with book_form_node_form_alter(), which is invoked for the node edit form of every content type.

As side note, the hook_forms() purpose is providing the same form builder for a group of forms whose ID follow a schema, such as in the case of comment edit forms, whose IDs are comment_node_[node type]_form. When provided, the form builder callback (passed as callback value) should build the full form, not part of it.
The fact my_dossier_form() builds just a submission button, and my_dossier_forms() is using the same code used by node_forms() makes me think the code purpose is altering the node edit forms, which should be accomplished by implementing hook_form_BASE_FORM_ID_alter(). That's what the Book module does with book_form_node_form_alter(), which is invoked for the node edit form of every content type.