Composer no longer available for PRE_COMMANDS in v7.0.x

[iisisrael]

Describe the bug
Vendor packages need to be installed for PHPStan to analyze use of injected dependencies, and a composer install pre-command fails with a command not found error in v7.0.x.

To Reproduce
Steps to reproduce the behavior:

1. Configure a PRE_COMMAND in .mega-linter.yml

PRE_COMMANDS:
  - command: composer install --prefer-dist --no-ansi --no-interaction --no-progress --no-scripts --ignore-platform-req=ext-sockets --ignore-platform-req=ext-intl --ignore-platform-req=ext-fileinfo --ignore-platform-req=ext-sodium --ignore-platform-req=ext-posix --ignore-platform-req=ext-gd --ignore-platform-req=ext-xml
    cwd: "workspace"

2. Run version 7: npx mega-linter-runner -r v7.0.3
3. Note the output:

The MegaLinter documentation can be found at:
 - https://megalinter.io/7.0.3
----------------------------------------------------------------------------------------------------
MegaLinter initialization
MegaLinter will analyze workspace [/tmp/lint]

[Pre] run: [composer install --prefer-dist --no-ansi --no-interaction --no-progress --no-scripts --ignore-platform-req=ext-sockets --ignore-platform-req=ext-intl --ignore-platform-req=ext-fileinfo --ignore-platform-req=ext-sodium --ignore-platform-req=ext-posix --ignore-platform-req=ext-gd --ignore-platform-req=ext-xml] in cwd [/tmp/lint]
[Pre] error:
/bin/bash: line 1: composer: command not found

4. Repeat on verson 6: npx mega-linter-runner -r v6.22.2
5. Note the output:

The MegaLinter documentation can be found at:
 - https://megalinter.io/6.22.2
----------------------------------------------------------------------------------------------------
MegaLinter initialization

[Pre] run: [composer install --prefer-dist --no-ansi --no-interaction --no-progress --no-scripts --ignore-platform-req=ext-sockets --ignore-platform-req=ext-intl --ignore-platform-req=ext-fileinfo --ignore-platform-req=ext-sodium --ignore-platform-req=ext-posix --ignore-platform-req=ext-gd --ignore-platform-req=ext-xml] in cwd [/tmp/lint]
[Pre] Installing dependencies from lock file (including require-dev)

Expected behavior
Composer should install vendor packages on a pre-command.

[bdovaz]

Composer was removed when PHPLint was upgraded because it was the only one that used it and it was no longer needed in that case:

https://github.com/oxsecurity/megalinter/pull/2638/files#diff-ec327cc33365f46316d29b37a7ec42324bd935ed7114c8f9ea9c7febf1a10a83

@llaville who is the author of the PHPLint upgrade to v9 commented it, I have no experience in PHP linters so I followed his steps: #2483

cc @nvuillam @echoix @Kurt-von-Laven

[nvuillam]

@iisisrael I think if you add a previous PRE_COMMAND with apk add composer it should be ok :) (that's how we used to install it before removing it)

@bdovaz > Good catch, thanks for mentioning :)

[iisisrael]

Thanks @nvuillam, that works.

[nvuillam]

@iisisrael I'm glad your use case is solved :)