[Bug]: previewMainTemplate Option Ignored, Default Template Used Instead

[Shadowrunner11]

Describe the bug

In the Storybook configuration, there is an option called previewMainTemplate which is intended to allow users to override the default template.ejs used by the Storybook build-manager when creating the main index.html of the standalone application.

However, when a custom path to an .ejs template is provided for the previewMainTemplate option and either storybook build or storybook dev is run, the custom template is not utilized as expected. Instead, the build process continues to use the default template.ejs , ignoring the previewMainTemplate configuration.

Reproduction link

https://stackblitz.com/edit/github-sbmzvp?file=.storybook%2FcustomIndex.ejs&view=editor

Reproduction steps

1. Go to the link above
2. Wait for the execution of yarn build-storybook or execute it manually in the integrated terminal
3. Open the generated storybook-static/index.html a
4. Compare the mentioned file with .storybook/customIndex.ejs template if the correct changes are being applied from the template to the static html file.

System

Storybook Environment Info:

  System:
    OS: macOS 14.2
    CPU: (12) arm64 Apple M2 Pro
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.19.1 - ~/.nvm/versions/node/v18.19.1/bin/node
    npm: 10.2.4 - ~/.nvm/versions/node/v18.19.1/bin/npm
    pnpm: 8.9.0 - ~/Library/pnpm/pnpm <----- active
  Browsers:
    Chrome: 122.0.6261.129
    Safari: 17.2
  npmPackages:
    @storybook/addon-essentials: ^8.1.10 => 8.1.10 
    @storybook/addon-interactions: ^8.1.10 => 8.1.10 
    @storybook/addon-links: ^8.1.10 => 8.1.10 
    @storybook/addon-onboarding: ^8.1.10 => 8.1.10 
    @storybook/blocks: ^8.1.10 => 8.1.10 
    @storybook/react: ^8.1.10 => 8.1.10 
    @storybook/react-vite: ^8.1.10 => 8.1.10 
    @storybook/test: ^8.1.10 => 8.1.10 
    eslint-plugin-storybook: ^0.8.0 => 0.8.0 
    storybook: ^8.1.10 => 8.1.10

Additional context

This is happening because in this line

storybook/code/builders/builder-manager/src/utils/data.ts

Line 18 in 56b17c6

const template = readTemplate('template.ejs');

the value of the the path to the template is being harcoded and resolved to the template.ejs of the library instead of resolving from the options.

In the meantime I provided a turn around while I work in the solution
#28301