How can a dynamic attribute from CI be passed to Docker during image build, for example, a project identifier, if environment variables are not used?

Build arguments can be used.

# Syntax for defining a build argument with a default value
ARG PROJECT_ID=default
# Using the argument in a COPY instruction, for example
COPY ./app_${PROJECT_ID}.jar /app/app.jar
# Or in a RUN instruction
RUN echo "Project ID is ${PROJECT_ID}"

In CI/CD pipelines, you can pass the argument value during image build using the --build-arg flag:

# Example build command in CI
docker build --build-arg PROJECT_ID=$CI_PROJECT_ID -t my-image:${CI_COMMIT_SHORT_SHA} .

Here, $CI_PROJECT_ID is a CI variable containing the project identifier.

Another method is to use a variable file for the build, which is more like an environment variable usage:

# Dockerfile remains unchanged
ARG PROJECT_ID=default
COPY ./app_${PROJECT_ID}.jar /app/app.jar

File build-args.env:

PROJECT_ID=my-specific-project

Build command with a file:

docker build --build-arg PROJECT_ID=$(cat build-args.env) -t my-image:${CI_COMMIT_SHORT_SHA} .

Another option is generating a file inside the Dockerfile using RUN:

ARG PROJECT_ID=default
# Generate a file with the project identifier
RUN echo "${PROJECT_ID}" > /app/project_id.txt
# The application can then use this file