Things I learned while writing a Dockerfile

I've been writing a Dockerfile for work that installs Postgresql and sets up a database. I ran into a few gotchas along the way. Here's the documentation of my misconceptions and the solutions.

One Line VS Multiple Lines

I originally was using one RUN command per shell line that I wanted to exectue:

Example:

RUN service postgresql start 
RUN su postgres -c "createuser -d -r -s root" 

The Problem

I wanted to separate the commands in order to optimize recreation of other images. But I'd always get the error that postgres wasn't running. That's exactly because each RUN command is a separate layer of running machine. Starting postgres in a RUN command does nothing because as soon as the next RUN command is reached, I'm in a new VM, and there's no postgres running at all.

The Solution

Join commands that are part of one cohesive set up step into one RUN statement:
(Note, thanks @outrunthewolf for pointing out that line continuations \ can be used to carry the same command across multiple lines for readability).

RUN service postgresql start && \
    su postgres -c "createuser -d -r -s root"

sudo VS su

The Problem

Online guides for installing and setting up postgres instruct the user to run commands as sudo, but this won't work with a Dockerfile because the commands are running as root, and root doesn't have access to the sudo command.

The Solution

Use su {USER} -c "commands go here" instead to execute commands as a different user.

An example Dockerfile

Here's an example Dockerfile that installs postgresql, creates a root role, and creates a database called accounts.

FROM ubuntu
MAINTAINER Murphy Randle, murphy@spacemonkey.com

## Install Postgresql deps
RUN apt-get install -y postgresql postgresql-contrib

## Postgresql setup:
RUN service postgresql start && su postgres -c "createuser -d -r -s root" && createdb accounts && service postgresql stop

ENTRYPOINT ["/bin/bash"]