copy.c

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>

int cp(const char*, const char*);

int main(int argc, char **argv) {
    if (argv[1] == NULL || argv[2] == NULL) {
      printf("Usage %s <source> <destination>\n", argv[0]);
      exit(2);
    }
    cp(argv[1], argv[2]);
}

// https://stackoverflow.com/q/2180079
int cp(const char *to, const char *from)
{
    int fd_to, fd_from;
    char buf[4096];
    ssize_t nread;
    int saved_errno;

    fd_from = open(from, O_RDONLY);
    if (fd_from < 0)
        return -1;

    fd_to = open(to, O_WRONLY | O_CREAT | O_EXCL, 0666);
    if (fd_to < 0)
        goto out_error;

    while (nread = read(fd_from, buf, sizeof buf), nread > 0)
    {
        char *out_ptr = buf;
        ssize_t nwritten;

        do {
            nwritten = write(fd_to, out_ptr, nread);

            if (nwritten >= 0)
            {
                nread -= nwritten;
                out_ptr += nwritten;
            }
            else if (errno != EINTR)
            {
                goto out_error;
            }
        } while (nread > 0);
    }

    if (nread == 0)
    {
        if (close(fd_to) < 0)
        {
            fd_to = -1;
            goto out_error;
        }
        close(fd_from);

        /* Success! */
        return 0;
    }

  out_error:
    saved_errno = errno;

    close(fd_from);
    if (fd_to >= 0)
        close(fd_to);

    errno = saved_errno;
    return -1;
}

Dockerfile

FROM frolvlad/alpine-glibc

COPY copy.c copy.c
RUN apk --no-cache add gcc libc-dev musl musl-dev && \
    gcc copy.c -static -o /bin/copy -Wno-implicit-function-declaration -static-libgcc && \
    ldd /bin/copy