Custom container for adding a new dependancy using Dockerfile returns error


I am trying to create a custom container for a software I already installed using Docker. I want to add some more dependancy and make another build in a separate container.
I got a documentation using Dockerfile to do this, - Create custom image for my project heading.

I followed the documentation to make a dockerfile


USER root
RUN apt-get -qq update &&
apt-get -y upgrade &&
apt-get -y install slepc-3.7 &&
apt-get clean &&
rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
USER root

but this results in an error,

Step 1/4 : FROM …/
—> ef9c8116655d
Step 2/4 : USER root
—> Using cache
—> ee1fcd919960
Step 3/4 : RUN apt-get -qq update && apt-get -y upgrade && apt-get -y install slepc-3.7 && apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
—> Running in 748b93b8840b
W: GPG error: …/…/t-lfs/ubuntu xenial InRelease: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 6B05F25D762E3157
W: The repository ‘…/…/…//github/git-lfs/ubuntu xenial InRelease’ is not signed.
Reading package lists…
Building dependency tree…
Reading state information…
Calculating upgrade…
The following packages have been kept back:
The following packages will be upgraded:
apt apt-transport-https apt-utils base-files bash binutils bsdutils bzip2
ca-certificates cpp-5 curl debconf dh-python distro-info-data dpkg dpkg-dev
e2fslibs e2fsprogs file g+±5 gcc-5 gcc-5-base gfortran-5 git git-lfs
git-man gnupg gpgv icu-devtools ifupdown iproute2 isc-dhcp-client
isc-dhcp-common libapparmor1 libapt-inst2.0 libapt-pkg5.0 libarchive13
libasan2 libatomic1 libaudit-common libaudit1 libblkid1 libbsd0 libbz2-1.0
libc-bin libc-dev-bin libc6 libc6-dev libcc1-0 libcilkrts5 libcomerr2
libcurl3 libcurl3-gnutls libdb5.3 libdbus-1-3 libdns-export162 libdpkg-perl
libexpat1 libexpat1-dev libfdisk1 libfreetype6 libgcc-5-dev libgcrypt20
libgfortran-5-dev libgfortran3 libglib2.0-0 libgnutls30 libgomp1
libgssapi-krb5-2 libhdf5-mpich-10 libhdf5-mpich-dev libicu-dev libicu55
libidn11 libisc-export160 libitm1 libjpeg-turbo8 libjpeg-turbo8-dev
libk5crypto3 libkmod2 libkrb5-3 libkrb5support0 liblcms2-2 libldap-2.4-2
liblsan0 libmagic1 libmount1 libmpx0 libnuma1 libobjc-5-dev libobjc4
libpam-doc libpam-modules libpam-modules-bin libpam-runtime libpam0g
libperl5.22 libpng12-0 libprocps4 libpython-dev libpython-stdlib
libpython2.7 libpython2.7-dev libpython2.7-minimal libpython2.7-stdlib
libpython3.5 libpython3.5-dev libpython3.5-minimal libpython3.5-stdlib
libquadmath0 libsasl2-2 libsasl2-modules-db libseccomp2 libsmartcols1
libsqlite3-0 libss2 libssl1.0.0 libstdc+±5-dev libstdc++6 libsystemd0
libtasn1-6 libtiff5 libtsan0 libubsan0 libudev1 libuuid1 libxml2
linux-libc-dev locales login mount multiarch-support openssh-client
openssh-server openssh-sftp-server openssl passwd patch perl perl-base
perl-modules-5.22 procps psmisc python python-apt-common python-cryptography
python-dev python-imaging python-minimal python-openssl python-pil
python-urllib3 python2.7 python2.7-dev python2.7-minimal python3-apt
python3-software-properties python3-urllib3 python3.5 python3.5-dev
python3.5-minimal rsync sensible-utils software-properties-common sudo
systemd systemd-sysv util-linux vim-common vim-tiny wget zlib1g zlib1g-dev
173 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
Need to get 197 MB of archives.
After this operation, 4975 kB of additional disk space will be used.
WARNING: The following packages cannot be authenticated!
E: There were unauthenticated packages and -y was used without --allow-unauthenticated
The command ‘/bin/sh -c apt-get -qq update && apt-get -y upgrade && apt-get -y install slepc-3.7 && apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*’ returned a non-zero code: 100

I have tried adding --allow-unauthenticated as well as adding sudo infront of the apt get lines…both doesn’t work. What exactly is the issue ? Could some one hep me solve this ?