Everytime docker successfully executes a RUN command from a Dockerfile, a new layer in the image filesystem is committed. Conveniently you can use those layers ids as images to start a new container.
Take the following Dockerfile:
FROM busybox
RUN echo 'foo' > /tmp/foo.txt
RUN echo 'bar' >> /tmp/foo.txt
and build it:
$ docker build -t so-2622957 .
Sending build context to Docker daemon 47.62 kB
Step 1/3 : FROM busybox
---> 00f017a8c2a6
Step 2/3 : RUN echo 'foo' > /tmp/foo.txt
---> Running in 4dbd01ebf27f
---> 044e1532c690
Removing intermediate container 4dbd01ebf27f
Step 3/3 : RUN echo 'bar' >> /tmp/foo.txt
---> Running in 74d81cb9d2b1
---> 5bd8172529c1
Removing intermediate container 74d81cb9d2b1
Successfully built 5bd8172529c1
You can now start a new container from 00f017a8c2a6, 044e1532c690and 5bd8172529c1:
$ docker run --rm 00f017a8c2a6 cat /tmp/foo.txt
cat: /tmp/foo.txt: No such file or directory
$ docker run --rm 044e1532c690 cat /tmp/foo.txt
foo
$ docker run --rm 5bd8172529c1 cat /tmp/foo.txt
foo
bar
of course you might want to start a shell to explore the filesystem and try out commands:
$ docker run --rm -it 044e1532c690 sh
/ # ls -l /tmp
total 4
-rw-r--r-- 1 root root 4 Mar 9 19:09 foo.txt
/ # cat /tmp/foo.txt
foo
When one of the Dockerfile command fails, what you need to do is to look for the id of the preceding layer and run a shell in a container created from that id:
docker run --rm -it <id_last_working_layer> bash -il
Once in the container:
- try the command that failed, and reproduce the issue
- then fix the command and test it
- finally update your Dockerfile with the fixed command
