How to debug image build with Dockerfile
Dockerfile to build the env for Ruby on Rails App
FROM ruby
# install essential libs/commands
RUN apt-get update && apt-get install lib-ssl
...
# install gems
RUN bundle install
...
# install npm
RUN yarn install
COMMAND bundle exec rails server
- delete the following part causing errors
FROM busybox
RUN echo 'hello world' > /tmp/test
RUN exit 1
....
RUN echo 'ready'
RUN exit 1
Just remove RUN exit 1 and the following
- run intermediate image with sha
Turn off buildkit to get the sha for each layer
docker run --rm -it current shto debug
❯ DOCKER_BUILDKIT=0 docker build -t test .
Sending build context to Docker daemon 2.048kB
Step 1/3 : FROM busybox
---> abaa813f94fd
Step 2/3 : RUN echo 'hello world'
---> Using cache
---> 551ba8324834
Step 3/3 : RUN exit 1
---> Running in 5e3c523c38a3
The command '/bin/sh -c exit 1' returned a non-zero code: 1
# pick sha of last successful layer
docker run --rm -it 551ba8324834 sh
-
nsenterto debug
Enter into the name space of the process.
Namespaces are a feature of the Linux kernel that partitions kernel resources such that one set of processes sees one set of resources while another set of processes sees a different set of resources.
- add
RUN sleep infiniteto Dockerfile -
docker run -it --rm --privileged --pid=host justincormack/nsenter1to get to the building host 1 -
ps -ef|grep sleepto find pid -
nsenter -p -m -u -i -n -t 10012 sh
FROM busybox
RUN echo 'hello world'
RUN sleep infinite
RUN exit 1
❯ docker build -t test .
[+] Building 10.7s (5/7)
=> [internal] load build definition from Dockerfile 0.0s
=> => transferring dockerfile: 73B 0.0s
=> [internal] load .dockerignore 0.0s
=> => transferring context: 2B 0.0s
=> [internal] load metadata for docker.io/library/busybox:latest 0.0s
=> [1/4] FROM docker.io/library/busybox 0.0s
=> CACHED [2/4] RUN echo 'hello world' 0.0s
=> [3/4] RUN sleep infinite
- use
targetto build sucessfully the image multiple-stage builds
FROM busybox as working
RUN echo 'hello world'
FROM working as error
RUN exit 1
# build image with target
docker build -t test --target working .
# run image to debug
docker run --rm -it test sh
docker run --privileged --pid=host -it alpine \
nsenter -t 1 -m -u -n -i sh
- --rm : removes the container after it is stopped -ti (or -t -i) : adds a tty and leaves the standard input opened
- --privileged : grants additional permissions to the container, it allows the container to gain access to the devices of the host (/dev)
- --pid=host : allows the containers to use the processus tree of the Docker host (the VM in which the Docker daemon is running)
check pid
❯ docker run -ti --rm busybox sh
❯ docker ps
❯ docker inspect --format '{{.State.Pid}}' a57c56a83e54
18762
-
althernative way using alpine and nsenter command ↩
Top comments (0)