The %post section in Apptainer .def files runs as /bin/sh by default, which does not support Bash arrays ((...) and ${array[@]}).

Fixes

Option 1: Use Bash explicitly

Add a shebang as the first line of %post:

%post
+ #!/bin/bash
    apt update
    packages=(
        htop
        vim
    )
    apt install -y "${packages[@]}"

Option 2: POSIX-compatible (no arrays)

%post
    apt update
-   packages=(
-       htop
-       vim
-   )
-   apt install -y "${packages[@]}"
+   apt install -y htop vim

or with a space-separated string:

%post
    apt update
+   packages="htop vim"
+   apt install -y $packages

Use Option 1 if you need arrays; otherwise, Option 2 is simpler and more portable.