powershell, 78 bytes

Result

AC

Duration

2011ms

Code [DL]

$t,$s=$input|%{$_.TrimEnd()}
"$t
"*$s.Count+$s[-1].PadRight($t.Length,75)|echo

stdin

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                      K                           

stdout

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                      KKKKKKKKKKKKKKKKKK

stderr

strace

10    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7ffc4b66a170 /* 6 vars */) = 0
12    execve("/usr/bin/pwsh", ["/usr/bin/pwsh", "/volume/CODE"], 0x55a61c2c4c28 /* 6 vars */ <unfinished ...>
11    execve("/bin/cat", ["cat", "-"], 0x55a61c2c4c10 /* 6 vars */ <unfinished ...>
12    <... execve resumed> )            = 0
11    <... execve resumed> )            = 0
11    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=11, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
15    +++ exited with 0 +++
16    +++ exited with 0 +++
12    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=15, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
30    +++ exited with 0 +++
23    +++ exited with 0 +++
27    +++ exited with 0 +++
28    +++ exited with 0 +++
26    +++ exited with 0 +++
29    +++ exited with 0 +++
25    +++ exited with 0 +++
24    +++ exited with 0 +++
22    +++ exited with 0 +++
21    +++ exited with 0 +++
20    +++ exited with 0 +++
19    +++ exited with 0 +++
17    +++ exited with 0 +++
18    +++ exited with 0 +++
14    +++ exited with 0 +++
13    +++ exited with 0 +++
12    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12, si_uid=0, si_status=0, si_utime=45, si_stime=24} ---
10    +++ exited with 0 +++