How to pass a multiline argument to Dockerfile RUN to one of the commands?

Question

Docker

For example, the command to issue a self-signed certificate uses a single-line config separated by a newline:

openssl req -x509 -out localhost.crt -keyout localhost.key \
  -newkey rsa:2048 -nodes -sha256 \
  -subj '/CN=localhost' -extensions EXT -config <( \
   printf "[dn]\nCN=localhost\n[req]\ndistinguished_name = dn\n[EXT]\nsubjectAltName=DNS:localhost\nkeyUsage=digitalSignature\nextendedKeyUsage=serverAuth")

see argument -config

I would like to put a similar instruction in my Dockerfile, which will pass a multiline text as one of the arguments to the called command.

Copy-paste throws an error/bin/sh: syntax error: unexpected "("

Dockerfile

RUN openssl req -x509 \
    -newkey rsa:2048 -nodes -sha256 \
    -out /etc/cert/localhost.crt  \
    -keyout /etc/cert/localhost.key \
    -subj '/CN=localhost' \
    -extensions EXT \
    -config < ( \
      printf '[dn]\nCN=localhost\n[req]\ndistinguished_name = dn\n[EXT]\nsubjectAltName=DNS:localhost\nkeyUsage=digitalSignature\nextendedKeyUsage=serverAuth')

The latest versions of the builder have the Heredoc syntax, but I would like it to work on older versions of Docker as well. How to pass the unpacked text as an argument to the command in RUN

from one line with characters ?\n

Answer 1

You have a space between "<" and "(", but you need "<("
And sh can't "<( some command )"
Dockerfile:

FROM ubuntu:latest

RUN apt update && apt install -y openssl

RUN mkdir -p /etc/cert

RUN /bin/bash -c "openssl req -x509 \
    -newkey rsa:2048 -nodes -sha256 \
    -out /etc/cert/localhost.crt  \
    -keyout /etc/cert/localhost.key \
    -subj '/CN=localhost' \
    -extensions EXT \
    -config <( \
      printf '[dn]\nCN=localhost\n[req]\ndistinguished_name = dn\n[EXT]\nsubjectAltName=DNS:localhost\nkeyUsage=digitalSignature\nextendedKeyUsage=serverAuth')"

docker build .

Sending build context to Docker daemon  2.048kB
Step 1/4 : FROM ubuntu:latest
 ---> 1318b700e415
Step 2/4 : RUN apt update && apt install -y openssl
 ---> Using cache
 ---> e608bba6cb59
Step 3/4 : RUN mkdir -p /etc/cert
 ---> Using cache
 ---> b89d4c7495a3
Step 4/4 : RUN /bin/bash -c "openssl req -x509     -newkey rsa:2048 -nodes -sha256     -out /etc/cert/localhost.crt      -keyout /etc/cert/localhost.key     -subj '/CN=localhost'     -extensions EXT     -config <(       printf '[dn]\nCN=localhost\n[req]\ndistinguished_name = dn\n[EXT]\nsubjectAltName=DNS:localhost\nkeyUsage=digitalSignature\nextendedKeyUsage=serverAuth')"
 ---> Running in a4012f6c4893
Generating a RSA private key
................................................+++++
..+++++
writing new private key to '/etc/cert/localhost.key'
-----
Removing intermediate container a4012f6c4893
 ---> 6ea49c8d9ada
Successfully built 6ea49c8d9ada