Source code and build instructions

The next step is to locate all the source code and other assets, such as images and CSS and HTML files that are part of the application. Ideally, they should be located in a single folder. This folder will be the root of our project and can have as many subfolders as needed. This project root folder will be the context during the build of the container image we want to create for our legacy application. Remember, the Docker builder only includes files in the build that are part of that context; in our case, that is the root project folder.

There is, though, an option to download or copy files during the build from different locations, using the COPY or ADD commands. Please refer to the online documentation for the exact details on how to use these two commands. This option is useful if the sources for your legacy application cannot be easily contained in a single, local folder.

Once we are aware of all the parts that are contributing to the final application, we need to investigate how the application is built and packaged. In our case, this is most probably done by using Maven. Maven is the most popular build automation tool for Java, and has been—and still is—used in most enterprises that are developing Java applications. In the case of a legacy .NET application, it is most probably done by using the MSBuild tool; and in the case of a C/C++ application, Make would most likely be used.

Once again, let's extend our inventory and write down the exact build commands used. We will need this information later on when authoring the Dockerfile.