There is already a question, with answer, under a similar title, but the title isn't really outlining the true question. mkdir is in fact executing for the other question/thread. It is just failing...

Whereas I've:

RUN mkdir -p /home/developer
And I get:
Step 18 : RUN MKDIR 
Unknown instruction: RUN MKDIR

That is ACTUALLY not executing.

Here is the Docker file from line 1. All of these steps succeed, until mkdir.

```
FROM ubuntu:14.04
ENV DEBIAN_FRONTEND=noninteractive
RUN apt-get -y purge php.*
RUN apt-get update
RUN apt-get upgrade -y
RUN apt-get install -y wget
RUN apt-get install -y build-essential
RUN wget http://xmlsoft.org/sources/libxml2-2.8.0.tar.gz
RUN tar -xvf libxml2-2.8.0.tar.gz
WORKDIR libxml2-2.8.0
RUN ./configure && make && make install
RUN apt-get install -y make
# build php
RUN wget http://museum.php.net/php5/php-5.4.0.tar.bz2
RUN tar -xvf php-5.4.0.tar.bz2
WORKDIR php-5.4.0
RUN ./configure && make
RUN make install
# Replace 1000 with your user / group id
RUN export uid=1000 gid=1000
# −p, −−parents -> no error if existing, make parent directories as needed
RUN mkdir -p /home/developer

```