Add files to system image in Yocto

I try to add two files to my system image using Yocto. I use the following code that does not give errors and is added to the conf/local.conf configuration file:

LICENSE = "MIT"
LIC_FILES_CHKSUM = "file://${COMMON_LICENSE_DIR}/MIT;md5=0835ade698e0bcf8506ecda2f7b4f302"

SRC_URI += " 
    file://rootfs.tar.gz;unpack=0 
    file://Dockerfile 
"

do_install () {
    install -d ${D}${datadir}
    install -m 755 ${WORKDIR}/rootfs.tar.gz ${D}${datadir}
    install -m 755 ${WORKDIR}/Dockerfile ${D}${datadir}
}

FILES_${PN} += " 
        ${datadir}/rootfs.tar.gz 
        ${datadir}/Dockerfile 
"

However, my files are not present in the generated root filesystem, do you have any ideas why ? I already read a lot of posts on different forums but none of the solution worked.

Source: StackOverflow