Multiple FROMs - what it means

Question

I want to build a docker image for the Linkurious project on github  which requires both the Neo4j database  and Node js to run   my first approach was to declare a base image for my image  containing Neo4j   The reference docs do not define  base image  in any helpful manner      Base image    An image that has no parent is a base image   from which I read that I may only have a base image if that image has no base image itself   but what is a base image  does it mean that if I declare neo4j neo4j in a FROM directive  that when my image is run the neo database will automatically run and be available within the container on port 7474   reading the Docker reference  see  https   docs docker com reference builder  from  I see       FROM can appear multiple times within a single Dockerfile in order to create multiple images  Simply make a note of the last image ID output by the commit before each new FROM command    do I want to create multiple images  it would seem what I want is to have a single image that contains the contents of other images e g  neo4j and node js  I ve found no directive to declare dependencies in the reference manual   are there no dependencies like in RPM where in order to run my image the calling context must first install the images it needs   I m confused

User · Accepted Answer

As of May 2017  multiple FROMs can be used in a single Dockerfile  See  quot Builder pattern vs  Multi-stage builds in Docker quot   by Alex Ellis  and PR 31257 by T  nis Tiigi   The general syntax involves adding FROM additional times within your Dockerfile - whichever is the last FROM statement is the final base image  To copy artifacts and outputs from intermediate images use COPY --from  lt base image number gt    FROM golang 1 7 3 as builder WORKDIR  go src github com alexellis href-counter  RUN go get -d -v golang org x net html   COPY app go      RUN CGO ENABLED 0 GOOS linux go build -a -installsuffix cgo -o app    FROM alpine latest   RUN apk --no-cache add ca-certificates WORKDIR  root  COPY --from builder  go src github com alexellis href-counter app      CMD   quot   app quot      The result would be two images  one for building  one with just the resulting app  much  much smaller  REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE  multi               latest              bcbbf69a9b59        6 minutes ago       10 3MB   golang              1 7 3               ef15416724f6        4 months ago        672MB      what is a base image   A set of files  plus EXPOSE d ports  ENTRYPOINT and CMD  You can add files and build a new image based on that base image  with a new Dockerfile starting with a FROM directive  the image mentioned after FROM is  quot the base image quot  for your new image   does it mean that if I declare neo4j neo4j in a FROM directive  that when my image is run the neo database will automatically run and be available within the container on port 7474   Only if you don t overwrite CMD and ENTRYPOINT  But the image in itself is enough  you would use a FROM neo4j neo4j if you had to add files related to neo4j for your particular usage of neo4j

User · Answer

The first answer is too complex  historic  and uninformative for my tastes   It s actually rather simple  Docker provides for a functionality called multi-stage builds the basic idea here is to   Free you from having to manually remove what you don t want  by forcing you to whitelist what you do want  Free resources that would otherwise be taken up because of Docker s implementation   Let s start with the first  Very often with something like Debian you ll see  RUN apt-get update       amp  amp  apt-get dist-upgrade      amp  amp  apt-get install  lt whatever gt       amp  amp  apt-get clean  We can explain all of this in terms of the above  The above command is chained together so it represents a single change with no intermediate Images required  If it was written like this  RUN apt-get update   RUN apt-get dist-upgrade  RUN apt-get install  lt whatever gt   RUN apt-get clean   It would result in 3 more temporary intermediate Images  Having it reduced to one image  there is one remaining problem  apt-get clean doesn t clean up artifacts used in the install  If a Debian maintainer includes in his install a script that modifies the system that modification will also be present in the final solution  see something like pepperflashplugin-nonfree for an example of that   By using a multi-stage build you get all the benefits of a single changed action  but it will require you to manually whitelist and copy over files that were introduced in the temporary image using the COPY --from syntax documented here  Moreover  it s a great solution where there is no alternative  like an apt-get clean   and you would otherwise have lots of un-needed files in your final image  See also  Multi-stage builds COPY syntax

[docker] Multiple FROMs - what it means

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