In the latest version of Docker Desktop a new command has been included call docker debug.

Now this is only available with a Pro Docker licence but it’s an interesting command so I thought I’d run through what it can do here.

Typically when I’m testing SQL Server in containers I build my own images as sometimes I need tools that aren’t available in the sql images from the microsoft container registry.

One example of this is ping. When spinning up containers on a custom bridge network all containers on that network should be able to reference each other by container name (containers on the default bridge network can only communicate by IP address).

In order to confirm (or troubleshoot) connectivity between the containers, I built a custom image with ping installed.

But with the new docker debug command I no longer have to.

What docker debug does is open up a shell to any container (or image) with a whole load of tools available. You can also install more tools without affecting the running container!

So effectively, this replaces the need to exec into a container and install the tools there.

Let’s have a look at this in action. I’m going to spin up two SQL containers (from WSL on Windows) on custom brigde network.

So first thing to do is create the network: –

docker network create sqlserver

And then spin up the containers (using the SQL Server 2022 image in the MCR):-

docker container run -d \
--publish 15789:1433 \
--network sqlserver \
--env ACCEPT_EULA=Y \
--env MSSQL_SA_PASSWORD=Testing1122 \
--name sqlcontainer1 \
mcr.microsoft.com/mssql/server:2022-CU11-ubuntu-20.04

docker container run -d \
--publish 15790:1433 \
--network sqlserver \
--env ACCEPT_EULA=Y \
--env MSSQL_SA_PASSWORD=Testing1122 \
--name sqlcontainer2 \
mcr.microsoft.com/mssql/server:2022-CU11-ubuntu-20.04

Ok, once they’re up and running…let’s use docker debug to connect to sqlcontainer1: –

sudo docker debug sqlcontainer1

Note – using sudo here as it’ll throw an error in WSL without it

And now let’s try to ping the other container by name: –

ping sqlcontainer2 -c 4

Cool! That worked, so I’ve confirmed that my containers can talk to each other by name on my custom bridge network.

Docker debug also comes with a few custom tools, one of which is entrypoint. This let’s us see what the ENTRYPOINT and CMD statements are of the underlying image the container was built from. Let’s check it out with our sql container: –

entrypoint

Nice, ok that’ll be really handy when debugging!

All in all, this is a very useful tool when working with containers. It’ll help keep our container images as small as possible because, let’s be honest, as container images go…the sql server image are huge!

Thanks for reading!