Docker build fails "COPY failed: no source files were specified"

Question

I'm new to Docker (but not to developing).

I have this Dockerfile:

    #
    # Build stage
    #
    FROM maven:3.6.3-jdk-8-slim AS build
    COPY src /home/app/src
    COPY pom.xml /home/app
    RUN mvn -f /home/app/pom.xml clean package

    #
    # Package stage
    #
    FROM openjdk:8-jdk-alpine
    RUN addgroup -S spring && adduser -S spring -G spring
    USER spring:spring
    ARG war_FILE=/home/app/target/*.war
    COPY ${war_FILE} app.war
    ENTRYPOINT ["java","-jar","/app.war"]

And (so far so good) the build succeeds:

    [INFO] ------------------------------------------------------------------------
    [INFO] BUILD SUCCESS
    [INFO] ------------------------------------------------------------------------
    [INFO] Total time: 01:51 min
    [INFO] Finished at: 2020-01-15T08:43:54Z
    [INFO] ------------------------------------------------------------------------

but in the end, the docker build fails:

    Removing intermediate container 7c66e8b7dbed
    ---> 74b1c50c84ad
    Step 5/10 : FROM openjdk:8-jdk-alpine
    ---> a3562aa0b991
    Step 6/10 : RUN addgroup -S spring && adduser -S spring -G spring
    ---> Running in ca236cf9a705
    Removing intermediate container ca236cf9a705
    ---> 0c255ef5868f
    Step 7/10 : USER spring:spring
    ---> Running in 8452dcff6a8a
    Removing intermediate container 8452dcff6a8a
    ---> 6265f412699a
    Step 8/10 : ARG war_FILE=/home/app/target/*.war
    ---> Running in 3b54067b2cca
    Removing intermediate container 3b54067b2cca
    ---> c186c4a7e443
    Step 9/10 : COPY ${war_FILE} app.war
    COPY failed: no source files were specified

I'm probably doing something incredibly stupid but I don't know where to start looking...

Answer 1

In your Dockerfile,

ARG war_FILE=/home/app/target/*.war
COPY ${war_FILE} app.war

Due to some reason, the ${war_FILE} is not getting updated. Can you check whether .war files are present at the /home/app/target/ after the build?

Also, if the war_FILE is a constant, why not directly copying the files from that location? i.e.

COPY /home/app/target/*.war app.war

Again, if there will be multiple .war files this can be a problem

Answer 2

On a multistage build, data of each stage is not shared among stages.

Thus, indeed on build stage you have successfully created the war file under /home/app/target/.

However, on second stage this path does not exist, resulting in the reported error.

To solve this, replace second stage with:

FROM openjdk:8-jdk-alpine
RUN addgroup -S spring && adduser -S spring -G spring
USER spring:spring
COPY --from=build /home/app/target/war_name.war app.war
ENTRYPOINT ["java","-jar","/app.war"]

Example from official docs.