Using composer custom cache location in bitbucket pipeline

Stetson Lewis

Hello,

  I am trying to cache my composer dependencies between builds.  From what I have seen, I am using a custom composer cache location since the image does not use the default `~/.composer/cache` location.  Upon running the following script, I see in the build teardown section of the Composer install section that a cache is indeed being created: "Assembling contents of new cache 'composer-custom'".  However, on subsequent runs of the build, it installs fresh composer dependencies and does not read from the cache. 

Since I can't quite seem to get the code snippet to look nicely in this textarea, here is a gist of my file.

Stetson Lewis

After discussion with support, running the composer install script:
```
curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
```
on the image I was using: php:7.3-fpm, did in fact use the default composer cache location, `~/.composer/cache`.  By removing the custom-cache and using the default `caches: composer` option, I was able to get it to work.  There could have been other configuration issues that was causing it not to work previously.  Hope this helps