Thanks @wapperdude and @Surrogate
To clarify my question: the "Container" reference was in relation to the "fixed" height nature of the header whenever the overall object* is resized and not for the functionality of grouping other shapes together.
I have created this visual (using grouped shapes and manual grouping/ungrouping/regrouping before resizing!!!) to better illustrate what I am trying to do. Please see for reference numbers:
1.1.1 = Default object appearance (single line of text) and overall object handles.
1.2.1 = Showing the body area auto-height resizing with additional lines of text.
1.3.1 = Same as 1.2.1 but with additional lines added to the Header and Body area.
1.1.2 = 1.1.1 if the object is widened by dragging handles (text reflows and height auto adjusts).
1.2.2 = 1.2.1 if the object is widened by dragging handles (text reflows and height auto adjusts).
1.3.2 = 1.3.1 if the object is widened by dragging handles (text reflows and height auto adjusts).
1.1.3 = 1.1.1 if the object is narrowed by dragging handles (text reflows and height auto adjusts).
1.2.3 = 1.2.1 if the object is narrowed by dragging handles (text reflows and height auto adjusts).
1.3.3 = 1.3.1 if the object is narrowed by dragging handles (text reflows and height auto adjusts).
1.4.1 = Shape snapping back to the default size (max height = auto height) when content deleted from 1.3.1.
1.5.1 = Shape snapping back (or having vertical resize locked from manual edit) so the max or minimum height = auto height) when corner or top/bottom handles attempt to increase/decrease the height of 1.3.2.
Notes:
* I am not sure whether to call this an OBJECT, SHAPE or STENCIL???
I have tried this but got (a) unexpectant extra space beneath the text, (b) the formula being overriden when the the shape was manually resized and (c) did not work when grouping two boxes together:
https://superuser.com/questions/844750/how-to-have-an-auto-fit-content-textbox-in-visio-2013