The Duden (a dictionary of the Standard High German language) wouldn’t agree (translation):
Some people may not give much thought to the capitalisation of pronouns when addressing someone informally. However, if you would like to know more, you can find out about the capitalisation of du and ihr here.
As a rule, the forms of the pronoun du are written in lower case, as are the plural forms ihr. Lower case also applies to the forms of the possessive pronouns dein, euer and also to deinesgleichen, deinerseits, etc.
In letters (including postcards and similar), emails and text messages, however, you can also use upper case: Liebe Elly, wir wünschen dir/Dir viel Erfolg in deinem/Deinem neuen Amt als Vorsitzende des Bachvereins und euch/Euch allen viele renaturierte Bäche voller überbordend lebensfreudiger Bachflohkrebse. In general, the following applies: In all forms of text in which the author addresses the reader personally, you can capitalise du/Du and the words mentioned. This applies, for example, to notes in shared flats (Katinka, you are definitely not being dissed if you take out the rubbish sometimes), comments by teachers in their students’ notebooks (That was a really great idea, Steffi) and so on. In all these cases, we recommend capitalisation.
Conversely, this also means that in direct speech (e.g. in stories or novels), you and the other words mentioned are never capitalised, as the author is not addressing the reader: ‘Oh, you’re well integrated, laughed Moira, I mean, you fit in well with the landscape’ (Sylvie Schenk: Der Gesang der Haut). It is also not capitalised in advertisements or posters, editorials in youth magazines or in questionnaires (e.g. in written exam questions), as these are not personal addresses.
“In all forms of text in which the author addresses the reader personally, you can capitalise du/Du”. I would say that’s the case here.
I always capitalise ‘Du’ when addressing someone directly, as I consider this to be a form of politeness. But I’m fine with ‘du’ when we want it this way.