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‎beginners/dockerfile/onbuild.md

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Yes, there is only one such line. When constructing a mirror with this one-line `Dockerfile` in each project directory, the three lines of the previous base image `ONBUILD` will start executing, successfully copy the current project code into the image, and execute for this project. `npm install`, generate an application image.
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# Lab
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```
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# Dockerfile
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FROM busybox
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ONBUILD RUN echo "You won't see me until later"
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```
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# Docker build
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```
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docker build -t me/no_echo_here .
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Uploading context 2.56 kB
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Uploading context
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Step 0 : FROM busybox
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Pulling repository busybox
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769b9341d937: Download complete
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511136ea3c5a: Download complete
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bf747efa0e2f: Download complete
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48e5f45168b9: Download complete
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---> 769b9341d937
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Step 1 : ONBUILD RUN echo "You won't see me until later"
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---> Running in 6bf1e8f65f00
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---> f864c417cc99
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Successfully built f864c417cc9
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```
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Here the ONBUILD instruction is read, not run, but stored for later use.
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```
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# Dockerfile
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FROM me/no_echo_here
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```
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docker build -t me/echo_here .
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Uploading context 2.56 kB
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Uploading context
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Step 0 : FROM cpuguy83/no_echo_here
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# Executing 1 build triggers
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```
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Step onbuild-0 : RUN echo "You won't see me until later"
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---> Running in ebfede7e39c8
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You won't see me until later
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---> ca6f025712d4
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---> ca6f025712d4
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Successfully built ca6f025712d4
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```
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## Ubutu Rails
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```
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FROM ubuntu:12.04
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RUN apt-get update -qq && apt-get install -y ca-certificates sudo curl git-core
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RUN rm /bin/sh && ln -s /bin/bash /bin/sh
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RUN locale-gen en_US.UTF-8
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ENV LANG en_US.UTF-8
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ENV LANGUAGE en_US.UTF-8
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ENV LC_ALL en_US.UTF-8
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RUN curl -L https://get.rvm.io | bash -s stable
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ENV PATH /usr/local/rvm/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
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RUN /bin/bash -l -c rvm requirements
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RUN source /usr/local/rvm/scripts/rvm && rvm install ruby
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RUN rvm all do gem install bundler
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ONBUILD ADD . /opt/rails_demo
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ONBUILD WORKDIR /opt/rails_demo
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ONBUILD RUN rvm all do bundle install
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ONBUILD CMD rvm all do bundle exec rails server
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```
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This Dockerfile is doing some initial setup of a base image. Installs Ruby and bundler. Pretty typical stuff. At the end are the ONBUILD instructions.
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ONBUILD ADD . /opt/rails_demo Tells any child image to add everything in the current directory to /opt/railsdemo. Remember, this only gets run from a child image, that is when another image uses this one as a base (or FROM). And it just so happens if you look in the repo I have a skeleton rails app in railsdemo that has it's own Dockerfile in it, we'll take a look at this later.
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ONBUILD WORKDIR /opt/rails_demo Tells any child image to set the working directory to /opt/rails_demo, which is where we told ADD to put any project files
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ONBUILD RUN rvm all do bundle install Tells any child image to have bundler install all gem dependencies, because we are assuming a Rails app here.
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ONBUILD CMD rvm all do bundle exec rails server Tells any child image to set the CMD to start the rails server
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Ok, so let's see this image build, go ahead and do this for yourself so you can see the output.
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```
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git clone [email protected]:sangam14/docker_onbuild.git
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cd docker_onbuild
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docker build -t sangam14/docker_onbuild .
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Step 0 : FROM ubuntu:12.04
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---> 9cd978db300e
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Step 1 : RUN apt-get update -qq && apt-get install -y ca-certificates sudo curl git-core
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---> Running in b32a089b7d2d
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# output supressed
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ldconfig deferred processing now taking place
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---> d3fdefaed447
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Step 2 : RUN rm /bin/sh && ln -s /bin/bash /bin/sh
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---> Running in f218cafc54d7
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---> 21a59f8613e1
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Step 3 : RUN locale-gen en_US.UTF-8
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---> Running in 0fcd7672ddd5
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Generating locales...
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done
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Generation complete.
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---> aa1074531047
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Step 4 : ENV LANG en_US.UTF-8
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---> Running in dcf936d57f38
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---> b9326a787f78
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Step 5 : ENV LANGUAGE en_US.UTF-8
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---> Running in 2133c36335f5
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---> 3382c53f7f40
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Step 6 : ENV LC_ALL en_US.UTF-8
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---> Running in 83f353aba4c8
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---> f849fc6bd0cd
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Step 7 : RUN curl -L https://get.rvm.io | bash -s stable
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---> Running in b53cc257d59c
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# output supressed
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---> 482a9f7ac656
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Step 8 : ENV PATH /usr/local/rvm/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
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---> Running in c4666b639c70
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---> b5d5c3e25730
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Step 9 : RUN /bin/bash -l -c rvm requirements
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---> Running in 91469dbc25a6
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# output supressed
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Step 10 : RUN source /usr/local/rvm/scripts/rvm && rvm install ruby
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---> Running in cb4cdfcda68f
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# output supressed
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Step 11 : RUN rvm all do gem install bundler
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---> Running in 9571104b3b65
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Successfully installed bundler-1.5.3
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Parsing documentation for bundler-1.5.3
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Installing ri documentation for bundler-1.5.3
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Done installing documentation for bundler after 3 seconds
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1 gem installed
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---> e2ea33486d62
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Step 12 : ONBUILD ADD . /opt/rails_demo
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---> Running in 5bef85f266a4
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---> 4082e2a71c7e
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Step 13 : ONBUILD WORKDIR /opt/rails_demo
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---> Running in be1a06c7f9ab
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---> 23bec71dce21
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Step 14 : ONBUILD RUN rvm all do bundle install
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---> Running in 991da8dc7f61
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---> 1547bef18de8
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Step 15 : ONBUILD CMD rvm all do bundle exec rails server
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---> Running in c49139e13a0c
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---> 23c388fb84c1
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Successfully built 23c388fb84c1
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```
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## Contributor - [Sangam Biradar](https://www.linkedin.com/in/sangambiradar14/)
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Next >> [healthcheck](https://dockerlabs.collabnix.com/beginners/dockerfile/healthcheck.html)

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