It is fairly common to build an application outside of Docker and inject the resulting binary. The most common example I see on SO is with Java-based applications. The Java class file format is designed to be portable across environments and so there aren’t a lot of differences if a .jar file is built on a developer’s workstation or not. You can run
mvn build
docker build -t myapp .
FROM tomcat:9
COPY target/myapp.war /usr/local/tomcat/apps
In this setup the target directory would be in .gitignore (you do not want build artifacts committed to source control) but it would not be in .dockerignore (it needs to be available to the image).
Some other patterns where this could be useful include:
- In a compiled language in a developer environment with somewhat long build times, so that you can get a test image out without spending several minutes waiting for
makein a multi-stage build - When an image needs to contain static assets that are hosted somewhere outside of source control, like Amazon S3
- If there are data sets that get generated at build time, that are large enough to not want to be checked in but small enough that adding them to the image is still practical (5-500 MB perhaps)
(I mostly ignore .dockerignore but I also try to be explicit about what files I COPY into my images.)