Workaround bash script to add a theme to the bitnami redmine-docker image (tested on macOS)

add-theme.sh

#!/bin/bash

REDMINE=$(docker-compose ps | grep redminedocker_redmine | awk '{ print $1 }')
echo -n "Enter your theme name: "
read THEME
if [ "$THEME" != "" ]; then
	echo $THEME
	docker exec -it $REDMINE ln -sf /bitnami/redmine/public/themes/$THEME /opt/bitnami/redmine/public/themes/
else
	echo "Please enter a theme name."
fi

Instructions:

Place your theme folder in /path/to/docker/project/redmine_data/public/themes/your_theme_directory

Put the script in the same directory as your docker project.

Make script executable:
chmod +x /path/to/add-theme.sh

Run the script:
./add-theme.sh

Follow the prompt and enter the name of your theme. If you have problems, check that the name of your image matches the grep in the script. In this gist that is named redminedocker_redmine

This has been tested on:
macOS 10.12.4,
Docker Community Version 17.03.1-ce-mac5 (16048)
Bitnami Docker Redmine 3.3.3-r0