Updated Docker apps (markdown)

Docker-apps.md

@@ -5,11 +5,17 @@ To do so:
 (say, particular versions of Linux and Python,
 with particular libraries and packages installed).
 
-* Package this environment as a Docker image:
+* Write a Dockerfile that builds the environment.
-i.e. write a Dockerfile that builds the environment.
+
+Note: your Docker image must include the ```ps``` command.
+Most Docker Linux images do, but for some reason the Debian image does not.
+If you use this image you'll need to add:
+```
+RUN apt-get update && apt-get install -y procps && rm -rf /var/lib/apt/lists/*
+```
 
 * Create BOINC app versions that combine your
-Docker image (and other files if needed) with a
+Dockerfile (and other files if needed) with a
 "Docker wrapper" program (supplied by BOINC) that
 interfaces between Docker and the BOINC client,
 running your jobs in Docker containers.
@@ -51,6 +57,13 @@ copy_to_container = [
 ```
 Before the job starts, copy the given file(s)
 (in the slot directory) to the given path in the container.
+
+Note: this does the same thing as putting
+```
+COPY in /app
+```
+in your Dockerfile.
+
 ```
 copy_from_container = [
     {src="/app/out", dst="out"}