There are some other alternative images exist already such as vimagick/mantisbt, xlrl/docker-mantisbt and a few others. Why do we need yet another image?

The reason is to combine all the nice features each of them have and add some missing features. To list some:

Always latest MantisBT version.
Comes with the latest PHP version (7.4 as for today)
Allows to easily configure presence of admin service folder
Comes with built-in integration with Gitlab and Github source plugins
Example docker-compose.yml file provided for getting started in one click!
Easy customization of the config files and custom plugins without destroying data from base image
Consistent Dockerfile style following all best practices (ensured by Dockerfile lint)

Quick start

Download docker-compose.yml from this repo: wget https://raw.githubusercontent.com/okainov/mantisbt-docker/master/docker-compose.yaml
Check the environment variables (at least you need to set MASTER_SALT env variable, doc)
docker-compose up -d
Open browser at localhost:8989/admin/install.php and follow installation instructions, default out-of-the-box values are good to use. -- Ignore Config File Exists but Database does not warning and proceed installation
Log in as administrator/root (default credentials) and confugre whatever you need (typically you want to create your own Admin user and disable built-in "administrator" first)
Check MantisBT own's checks at localhost:8989/admin/. Note: several warnings are expected to be "WARN" due to issues in MantisBT, such as magic quotes warning (#26964) and "folder outside of web root" warnings (#21584))
When ready to move to production, either remove MANTIS_ENABLE_ADMIN env variable or set it to 0 - this will remove "admin" folder from the installation.

For further details refer to official documentation

Extensions

Custom config settings

If you need to customize more options in config, create config_inc_addon.php file and mount it to /var/www/html/config/config_inc_addon.php in container. This fill will be added to default config_inc.php. Mounting it will allow you to see the changes instantly without rebuilding/restarting the container.

Some of the typical settings you might want to change:

$g_window_title = 'Title of your MantisBT instance';

// Default is useless 5 minutes
$g_reauthentication_expiry = 60 * 60;

// Enable anonymous access

$g_allow_anonymous_login = true;
$g_anonymous_account = 'anonymous';

Email

There are following env variables supported:

EMAIL_WEBMASTER - maps to g_webmaster_email
EMAIL_FROM - maps to g_from_email
EMAIL_RETURN_PATH - maps to g_return_path_email

Those are good enough to start with. Going further, to configure SMTP you might need to create custom config (as described above) with the values like:

$g_phpMailer_method = PHPMAILER_METHOD_SMTP;
$g_smtp_host = 'mail.domain.com';
$g_smtp_username = 'mail@domain.com';
$g_smtp_password = 'FILLME';

More details are available in official documentation

Custom plugins

In order to add your own custom plugins into the image, either create your own Dockerfile and copy extra plugins to /var/www/html/plugins/ or add volume in docker-compose to mount extra plugin directly inside existing image ./custom_plugin/:/var/www/html/plugins/custom_plugin/