Updating from ghost.30 to 3.42 in dockerfile

Developer help
1

I changed 3.0 to 3.42 in my dockerfile and now i get

FROM ghost:3.42.0

RUN apt-get update -y && \

apt-get install -y mysql-client

COPY ./init/laminarbackup.tar.gz /var/lib/ghost

COPY ./init/restorefiles.sh /var/lib/ghost

COPY ./init/ghost-starter-theme /var/lib/ghost/content/themes/ghost-starter-theme

COPY ./wait-for-it.sh /usr/local/bin/wait-for-it.sh

now I get

Package mysql-client is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package ‘mysql-client’ has no installation candidate
ERROR: Service ‘ghost’ failed to build: The command ‘/bin/sh -c apt-get update -y && apt-get install -y mysql-client’ returned a non-zero code: 100
And the script exits

any suggestions appreciated