install failed
Get:6 http://deb.debian.org/debian buster-updates/main amd64 Packages [7380 B]
Fetched 8343 kB in 3s (3226 kB/s)
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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
Package ssmtp 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 'ssmtp' has no installation candidate
E: Package 'mysql-client' has no installation candidate**
ERROR: Service 'app' failed to build: The command '/bin/sh -c apt-get update && apt-get install -y apache2 openssl php php-cli php-mysql php-curl php-fpm php-gd php-json php-xml php-zip php-mbstring software-properties-common git ssmtp mysql-client' returned a non-zero code: 100