WorkdirRelativePath: Relative workdir "app" can have unexpected results if the base image changes is a build check warning, not an error: the build still finished with exit code 0. It flags a WORKDIR without a leading slash. Write the absolute path, WORKDIR /app, and the warning goes away.

A relative WORKDIR is joined to whatever working directory the base image left behind. On alpine:latest that gave /app; on a base image that sets its own WORKDIR /app, the same line gave /app/app.

The error

#5 [2/3] WORKDIR app
#5 CACHED

#6 [3/3] RUN pwd
#6 0.192 /app
#6 DONE 0.2s
...
 1 warning found (use docker --debug to expand):
 - WorkdirRelativePath: Relative workdir "app" can have unexpected results if the base image changes (line 2)
exit code: 0

The terminal colour codes around "1 warning found" were removed from the capture above.

Why it happens

WORKDIR app means "the folder app inside the current working directory". In alpine:latest the current directory at the start of the stage is /, so the step printed /app and the image looked right. The same three lines on another base image, one built for this test from alpine:latest with WORKDIR /app, printed something else: 

#5 [2/3] WORKDIR app
...
#6 [3/3] RUN pwd
#6 0.224 /app/app

Only the FROM line changed, and the step after WORKDIR ran one folder deeper, in /app/app. That is the kind of "unexpected results" the warning names: the path depends on an image you may not control.

The fix

FROM alpine:latest
WORKDIR /app
RUN pwd
CMD ["sh"]

Rebuilt with docker build --progress=plain --no-cache -t fix9-bld-c8 ., the RUN pwd step printed /app, the build ended with exit code: 0 and no warning was printed. An absolute path gives the same folder whatever the base image's own WORKDIR is. If a relative path is what you want, for example to work below a folder the base image chose, the warning can be left in place; it does not fail the build.

How it was reproduced

A four-line Dockerfile on alpine:latest (image id 294b683cb724) with WORKDIR app, built with docker build --progress=plain -t fix9-bld-c8 ., then the same lines on a test image whose working directory is /app, built with docker build --progress=plain --no-cache -t fix9-bld-c8b .. Docker Desktop for Windows, client and engine 29.9.3, buildx v0.38.1. The image names are the test's own.

Frequently asked

Is WorkdirRelativePath an error in docker build?
No. It is a build check warning; in the test the build ended with exit code 0 and the image was created.
Why is a relative WORKDIR a problem in a Dockerfile?
It is joined to the base image's working directory. WORKDIR app gave /app on alpine and /app/app on a base image that had WORKDIR /app.
How do I fix the WorkdirRelativePath warning?
Write the path with a leading slash, such as WORKDIR /app. The rebuilt image printed /app and the warning was gone.

More decoded errors in the Fixes category. Another build check warning with a one-line fix is FromAsCasing.