Notes on Docker Course - Developer Beginner Linux Containers

I walked through the Developer Beginner Linux Containers course offered by Docker today, and hit a few snags. There's no obvious way to post comments or feedback to get the instructions updated, so here are my notes, for what they're worth.

Lesson 2.0 Webapps with Docker

This is the first snag:

$ sudo docker build -t bob_the_squirrel/myfirstapp .
[sudo] password for bob:
Sending build context to Docker daemon 8.192 kB
Step 1 : FROM alpine:latest
---> 88e169ea8f46
Step 2 : RUN apk add --update py-pip
---> Running in 15deabd22845
fetch http://dl-cdn.alpinelinux.org/alpine/v3.5/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.5/community/x86_64/APKINDEX.tar.gz
ERROR: unsatisfiable constraints:
 py-pip (virtual):
   provided by: py2-pip
   required by: world[py-pip]
The command '/bin/sh -c apk add --update py-pip' returned a non-zero code: 1

Easily fixed by modifying the Dockerfile:

# install the pip package and any dependencies
RUN apk add --update py2-pip # the lesson says to use py-pip


The next snag was this:

$ sudo docker build -t bob_the_squirrel/myfirstapp .
[sudo] password for bob:
Sending build context to Docker daemon 8.192 kB
Step 1 : FROM alpine:latest
---> 88e169ea8f46
Step 2 : RUN apk add --update py2-pip
---> Running in 33207dfe8efd
fetch http://dl-cdn.alpinelinux.org/alpine/v3.5/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.5/community/x86_64/APKINDEX.tar.gz
(1/12) Installing libbz2 (1.0.6-r5)
(2/12) Installing expat (2.2.0-r0)
(3/12) Installing libffi (3.2.1-r2)
(4/12) Installing gdbm (1.12-r0)
(5/12) Installing ncurses-terminfo-base (6.0-r7)
(6/12) Installing ncurses-terminfo (6.0-r7)
(7/12) Installing ncurses-libs (6.0-r7)
(8/12) Installing readline (6.3.008-r4)
(9/12) Installing sqlite-libs (3.15.2-r0)
(10/12) Installing python2 (2.7.13-r0)
(11/12) Installing py-setuptools (29.0.1-r0)
(12/12) Installing py2-pip (9.0.0-r0)
Executing busybox-1.25.1-r0.trigger
OK: 61 MiB in 23 packages
---> 840b9af43db6
Removing intermediate container 33207dfe8efd
Step 3 : COPY requirements.txt /usr/src/app
---> 0eac1e0f7a53
Removing intermediate container e9c368692b72
Step 4 : RUN pip install --no-cache-dir -r /usr/src/app/requirements.txt
---> Running in cd9acc93e4f3
Could not open requirements file: [Errno 20] Not a directory: '/usr/src/app/requirements.txt'
You are using pip version 9.0.0, however version 9.0.1 is available.

You should consider upgrading via the 'pip install --upgrade pip' command. 

The command '/bin/sh -c pip install --no-cache-dir -r
/usr/src/app/requirements.txt' returned a non-zero code: 1

Turns out this was my fault because I forgot to add a trailing slash to the Dockerfile COPY command:

# Add files that make up the Flask application
COPY requirements.txt /usr/src/app # oops
RUN pip install --no-cache-dir -r /usr/src/app/requirements.txt

If the trailing slash is included (/usr/src/app/), it looks like the COPY command will make the directory if it doesn't already exist.