With "software delivery" we mean the process of releasing your new software to its users. In our web development world this basically means bringing all parts of the software to a server, so the user can access the software with her browser.
So basically it is bundling up software, moving it to a server and starting it. We did something like this for decades.
Today's software consists of a lot of moving parts. This parts can be a back end app, a front end app, a database and all kinds of other components.
All those parts need to run on a server. And all those parts can use different technologies and use different libraries from that server. If you update one of those libraries on the server to a new version (because your back end app needs the new version) it can happen, that this breaks your database (because your database needs the old version of the library and can not work with the new version)
With Docker this problem is solved, because everything your software "parts" need is inside their container. The container does not care about the library on the server, it has its own version of the library inside. Encapsulation is the keyword here.
So you have started the new version of your container on the server and have stopped the old version. Great.
But it turns out that soon you get a lot of error in your log files and angry users are messaging you that "nothing is working anymore"!
On a server without Docker this can be a nightmare because you have to bring the server in the exact state as it was before the last release. And you have to do it fast! This can be really really tricky and it will be very stressful.
On a server with Docker you just stop the new version of your container and start the old version of your container again. Problem solved! (This assumes that you still have your old container lying around on your server)
Implementing Docker makes your software delivery easier and faster. It also makes deployments far easier to roll back. Removing stress from the product team and basically making deployments a non-event.
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