How do I reset the equation counter in a tcolorbox align environment?

There are various hooks that allow one to execute some code at various stages of the construction of the box. The following uses code and after. It also redefines and restores equation numbering, similar to the answers to this question. Note, however, that I do not want to make the impression that this is a super stable solution. I did not check whether this works with hyperref, nor did I do other cross-checks. Yet a conceivably more stable solution, which may or may not make use of future versions of the LaTeX kernel, may possibly use the same keys as in this code.

documentclass{article}
usepackage{amsmath}
usepackage{tcolorbox}
tcbuselibrary{theorems}
newcounter{savedeq}
newtcolorbox[auto counter]{derivation}{%
  code=setcounter{savedeq}{value{equation}}%
    setcounter{equation}{0}renewcommand{theequation}{thetcbcounter-arabic{equation}},%
  after=setcounter{equation}{value{savedeq}},%
  ams align,%
  title={DERIVATION thetcbcounter},%
  fonttitle=bfseries,%
  label={derivation@thetcbcounter},
  coltitle=black,%
  colback=orange!25,%
  colframe=orange!60,%
  %colback=orange!5!white,%
  %colframe=orange!75!black%
}%
%numberwithin{equation}{derivation}
begin{document}
begin{equation}
E=mc^2
end{equation}
begin{derivation}
  x + y &= z     && text{given} 
      y &= z - x && text{solve for (y)}
end{derivation}
begin{equation}
E=hnu
end{equation}
begin{derivation}
  gamma &= frac{1}{sqrt{1-v^2}}      && text{definition} 
       v &= sqrt{1-frac{1}{gamma^2}} && text{solve for (v)}
end{derivation}
begin{equation}
a^2+b^2=c^2
end{equation}
end{document}