"Deprecated function: implode()" on GMap Drupal module
This answer draws upon the comments above by leymannx. The essential problem is an incompatibility between the GMap module and PHP 7.4, for which a patch is available. In this answer the patch is applied to the code repository itself, prior to any Docker builds, this is possibly the cleanest method.
The first step is to download the patch into the right folder in the repository:
cd sites/all/modules/gmap
wget https://www.drupal.org/files/issues/2020-03-06/gmap-php7.4.patch

The patch file is encoded for MSDOS, therefore it must be converted before the patch can be applied. Here all the target files are converted too, as a precaution:
dos2unix ./gmap-php7.4.patch
dos2unix gmap_test/gmap_test.features.inc
dos2unix lib/Drupal/gmap/GmapDefaults.php
dos2unix tests/inc/gmap_defaults.inc

The patch can now be applied. Since this is a git repository, the git command itself can not be used.
patch -p1 -i ./gmap-php7.4.patch

Finally remove all the extra files the process may have created. They are not needed in the container.
rm *.patch gmap_test/gmap_test.features.inc.orig lib/Drupal/gmap/GmapDefaults.php.orig tests/inc/gmap_defaults.inc.orig