tikzpicture: make a foreach draw [yshift=n*u] stop at coordinate 1

I don't think, that you will truely need clip (-0.1,-0.1) rectangle (1.1,1.1); due to the work with tcolorbox.

But as an answer to the question "How to clip the y-axis":
You need to know, how many points "1", the image-height, is. And this is measured out in exactly the same way, as the "unitlength" was measured out in the code:

path[draw=none] (0,0) coordinate(LL) -- (0,1) coordinate(UL);
path let p1=($(UL)-(LL)$) in 
pgfextra{  pgfmathsetlengthmacro{ImageHeight}{veclen(x1,y1)}   }
[savevalue={h}{ImageHeight}];

foreach n in {0.1,0.2,...,10}{%%
pgfmathsetmacroYshift{n*u}
pgfmathsetmacroYshiftTest{n*u < h ?  1 : 0}
ifnumYshiftTest=1 draw[] ([yshift=Yshift]B) -- +(-1.5mm,0); elsefi
}%%
ifnumShowHelps=1
draw[red] (LL) circle(2pt) node[below]{LL};
draw[red] (UL) circle(2pt) node[above]{UL};
node[right=3mm, Help] at (UL) {Image Height h=h};    fi 

documentclass{article}

usepackage{mwe} % Dummy images
usepackage[skins]{tcolorbox}
tcbuselibrary{raster}

usepackage{tikz}
usepackage{tikzpagenodes} % current page text area.center
usetikzlibrary{calc} 
pgfkeys{/tikz/savevalue/.code 2 args={globaledef#1{#2}}}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

defShowHelps{1} %    1 'yes'        0   'no'

newcommand{UOneCoordinateX}{0.2}
newcommand{UOneCoordinateY}{0.09}
newcommand{UTwoCoordinateX}{0.2}
newcommand{UTwoCoordinateY}{0.284}

newcommand{GridSize}{0.05}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

begin{document}

begin{tikzpicture}[
remember picture, overlay, % page center 1/3
shift={(current page.center)}, % page center 2/3
font=sffamily,
Help/.style={font=large, text=red, fill=yellow}
]
node[anchor=center, %south west,
inner sep=0 pt] (myimage) at (0,0) {

begin{tcbitemize}[size=tight, halign=center, colback=gray, raster equal skip=0pt, raster left skip=0mm, raster right skip=0mm, raster width=1textwidth, boxrule = 0pt, frame hidden]
tcbitem includegraphics[width=textwidth]{example-image.jpg}
tcbitem includegraphics[width=textwidth]{example-image.jpg}
end{tcbitemize}  
};
begin{scope}[x={($2*(myimage.east)$)},y={($2*(myimage.north)$)},
shift={(myimage.south west)} % page center 3/3
]

% Restrict the draw area
clip (-0.1,-0.1) rectangle (1.1,1.1);
ifnumShowHelps=1 draw[red] (-0.1,-0.1) rectangle (1.1,1.1);fi

%%%%%ShowHelps
path[] (UOneCoordinateX,UOneCoordinateY) coordinate(U1) -- (UTwoCoordinateX,UTwoCoordinateY) coordinate(U2);
ifnumShowHelps=1 draw[red] (U1) circle(2pt) node[label=left:U1]{} -- (U2) circle(2pt) node[label=left:U2]{};fi % show unitlength line

path let p1=($(U2)-(U1)$) in 
pgfextra{  pgfmathsetlengthmacro{unitlength}{veclen(x1,y1)}   }
[savevalue={u}{unitlength}];
ifnumShowHelps=1 node[Help] at (0.3,0.5) {unitlength U1U2 = u}; fi%<--- show unitlength
ifnumShowHelps=1 draw[blue, transform canvas={xshift=2mm}] (U1) -- +(0,u);fi  % Test



% Image Ruler y
draw[very thick] (0,0) coordinate(B) -- (0, 1);
foreach n in {0,1,...,10}{%%
draw[very thick] ([yshift=n*u]B) -- +(-3mm,0) node[left]{
ifnumn=0 else ifnumn=1 n,cm else n fifi}; 
}%%

path[draw=none] (0,0) coordinate(LL) -- (0,1) coordinate(UL);
path let p1=($(UL)-(LL)$) in 
pgfextra{  pgfmathsetlengthmacro{ImageHeight}{veclen(x1,y1)}   }
[savevalue={h}{ImageHeight}];

foreach n in {0.1,0.2,...,10}{%%
pgfmathsetmacroYshift{n*u}
pgfmathsetmacroYshiftTest{n*u < h ?  1 : 0}
ifnumYshiftTest=1 draw[] ([yshift=Yshift]B) -- +(-1.5mm,0); elsefi
}%%
ifnumShowHelps=1
draw[red] (LL) circle(2pt) node[below]{LL};
draw[red] (UL) circle(2pt) node[above]{UL};
node[right=3mm, Help] at (UL) {Image Height h=h};    fi 
end{scope}
end{tikzpicture}

end{document}