Debug docker script more efficiently - docker

Question

I'm getting started on my first docker script and i'm trying to debug the last step and it's very slow debugging b/c the steps before it take a few minutes to run so if i have a typo, i have to re run the entire script.
Is there a more efficient way to debug a docker script? or do i have to rebuild the whole thing every time?
#FROM ubuntu:14.04
FROM node:0.10.40
#FROM mongo:2.6.11
# The port we're running the server on
EXPOSE 10645
# Set this as the working directory
WORKDIR /myproject/hapi
# Move the myproject files to /myproject in the docker container
ADD . /myproject/hapi
# Install the server dependencies
RUN pwd && ls -al && npm install
# Start everything up
CMD npm start
Log output:
^CR5033505:myproject m089269$ docker build -t myproject-hapi .
Sending build context to Docker daemon 932.2 MB
Step 0 : FROM node:0.10.40
---> a7d8016a6fdb
Step 1 : EXPOSE 10645
---> Running in ebc4f8ebbf7b
---> 701320586e6a
Removing intermediate container ebc4f8ebbf7b
Step 2 : WORKDIR /myproject/hapi
---> Running in 1998f97b252a
---> 1414baf38920
Removing intermediate container 1998f97b252a
Step 3 : ADD . /myproject/hapi
---> c80e665da20b
Removing intermediate container f6904fab79ce
Step 4 : RUN pwd && ls -al && npm install
---> Running in a3ef28ed70ae
/myproject/hapi
total 68
drwxr-xr-x 9 root root 4096 Oct 30 18:35 .
drwxr-xr-x 3 root root 4096 Oct 30 18:35 ..
-rw-r--r-- 1 root root 509 Apr 10 2015 .editorconfig
drwxr-xr-x 8 root root 4096 Oct 30 18:33 .git
-rw-r--r-- 1 root root 491 Oct 20 15:09 .gitignore
drwxr-xr-x 8 root root 4096 Aug 19 14:51 .idea
-rw-r--r-- 1 root root 1781 Apr 10 2015 .jscsrc
-rw-r--r-- 1 root root 6164 Apr 10 2015 .tfignore
-rw-r--r-- 1 root root 430 Oct 30 18:33 Dockerfile
-rw-r--r-- 1 root root 371 Oct 30 18:16 Dockerfile-client
-rwxr-xr-x 1 root root 1374 Oct 30 15:15 README.md
drwxr-xr-x 5 root root 4096 Oct 21 21:18 ab-testing-deploy
drwxr-xr-x 3 root root 4096 Oct 30 15:15 build
drwxr-xr-x 14 root root 4096 Oct 30 15:15 client
drwxr-xr-x 2 root root 4096 Apr 10 2015 githooks
drwxr-xr-x 10 root root 4096 Oct 30 15:15 hapi
npm ERR! install Couldn't read dependencies
npm ERR! Linux 4.1.10-boot2docker
npm ERR! argv "node" "/usr/local/bin/npm" "install"
npm ERR! node v0.10.40
npm ERR! npm v2.14.1
npm ERR! path /myproject/hapi/package.json
npm ERR! code ENOPACKAGEJSON
npm ERR! errno 34
npm ERR! package.json ENOENT, open '/myproject/hapi/package.json'
npm ERR! package.json This is most likely not a problem with npm itself.
npm ERR! package.json npm can't find a package.json file in your current directory.
npm ERR! Please include the following file with any support request:
npm ERR! /myproject/hapi/npm-debug.log
The command '/bin/sh -c pwd && ls -al && npm install' returned a non-zero code: 34

Answer 1

Unfortunately when you're doing your ADD command you're invalidating the docker build cache, so any commands after that will be run from scratch. From the docs:
Note: The first encountered ADD instruction will invalidate the cache for all following instructions from the Dockerfile if the contents of have changed. This includes invalidating the cache for RUN instructions.
https://docs.docker.com/reference/builder/#add
I think in this case it would be easiest to run an interactive container and run each command from your Dockerfile one by one so you can see which one fails and be in a position to run it again immediately after you attempt to fix the error.
For example:
docker run -ti -v /path/to/your/code:/myproject/hapi node:0.10.40 bash
$ cd /myproject/hapi
$ npm install
Then when you do an ls -la you'll be able to see that package.json either isn't there or something else is wrong, and once you've figured it out you can add that command into your Dockerfile.