Extending local Dockerfile - docker

Question

I'm trying to base a Dockerfile on another local one.
$ ls -lR
total 0
-rw-r--r-- 1 me me 42 14 avr 10:42 Dockerfile
drwxr-xr-x 3 me me 42 14 avr 10:42 prod
./prod:
total 0
-rw-r--r-- 1 me me 42 14 avr 10:42 Dockerfile
$ cat prod/Dockerfile
FROM ../Dockerfile
...
$ docker build - < prod/Dockerfile
unable to process Dockerfile: unable to parse repository info: repository name component must match "a-z0-9(?:[._]a-z0-9)*"
I know that FROM expects an image and not a path.
How can I extend Dockerfile from prod/Dockerfile ?

Answer 1

Dockerfiles don't extend Dockerfiles but images, the FROM line is not an "include" statement.
So, if you want to "extend" another Dockerfile, you need to build the original Dockerfile as an image, and extend that image.
For example;
Dockerfile1:
FROM alpine
RUN echo "foo" > /bar
Dockerfile2:
FROM myimage
RUN echo "bar" > /baz
Build the first Dockerfile (since it's called Dockerfile1, use the -f option as docker defaults to look for a file called Dockerfile), and "tag" it as myimage
docker build -f Dockerfile1 -t myimage .
# Sending build context to Docker daemon 3.072 kB
# Step 1 : FROM alpine
# ---> d7a513a663c1
# Step 2 : RUN echo "foo" > /bar
# ---> Running in d3a3e5a18594
# ---> a42129418da3
# Removing intermediate container d3a3e5a18594
# Successfully built a42129418da3
Then build the second Dockerfile, which extends the image you just built. We tag the resulting image as "myextendedimage";
docker build -f Dockerfile2 -t myextendedimage .
# Sending build context to Docker daemon 3.072 kB
# Step 1 : FROM myimage
# ---> a42129418da3
# Step 2 : RUN echo "bar" > /baz
# ---> Running in 609ae35fe135
# ---> 4ea44560d4b7
# Removing intermediate container 609ae35fe135
# Successfully built 4ea44560d4b7
To check the results, run a container from the image and verify that both files (/bar and /baz) are in the image;
docker run -it --rm myextendedimage sh -c "ls -la ba*"
# -rw-r--r-- 1 root root 4 Apr 14 10:18 bar
# -rw-r--r-- 1 root root 4 Apr 14 10:19 baz
I suggest to read the User guide, which explains how to work with images and containers

Answer 2

Take a look at multi-stage builds it could help you
https://docs.docker.com/engine/userguide/eng-image/multistage-build/
https://blog.alexellis.io/mutli-stage-docker-builds/

Answer 3

I wrote simple bash script for this. It works next way:
Example structure:
|
|_Dockerfile(base)
|_prod
|_Dockerfile(extended)
Dockerfile(extended):
FROM ../Dockerfile
...
Run script:
./script.sh prod
It merges your base dockerfile with extended and build merged file.
Script:
#!/bin/bash
fromLine=$(head -n 1 $1/Dockerfile)
read -a fromLineArray <<< $fromLine
extPath=${fromLineArray[1]}
tail -n +2 "$1/Dockerfile" > strippedDocker
cat $1/$extPath strippedDocker > resDocker
rm strippedDocker
docker build - < resDocker
rm resDocker

Answer 4

I'm using conditionals:
Dockerfile
Install sudo only on local build.
FROM ubuntu:latest
ARG APP_ENVIRONMENT=local
RUN apt-get update && bash -c "set -ex ; \
apt-get install -y $([ ${APP_ENVIRONMENT} = local ] \
&& echo 'curl sudo' \
|| echo 'curl' \
)"
CMD bash -c "set -ex ; \
[ ${APP_ENVIRONMENT} = local ] \
&& { app debug ; exit $? ; } \
|| { app start ; exit $? ; } \
"
Build
# Production
docker build \
-t my-image \
--build-arg APP_ENVIRONMENT='prod' \
.
# Local
docker build \
-t my-image \
.
Docker Compose
version: "3.7"
services:
app:
build:
context: .
args:
APP_ENVIRONMENT: "${APP_ENVIRONMENT:-local}"

Answer 5

If you use Docker 20.10+, you can do this:
# syntax = edrevo/dockerfile-plus
INCLUDE+ ../Dockerfile
RUN ...
The INCLUDE+ instruction gets imported by the first line in the Dockerfile. You can read more about the dockerfile-plus at https://github.com/edrevo/dockerfile-plus