Change workdir to /app

[rosshadden]

As I understand it /app is pretty much the canonical mount path. The Docker docs use this path as an example in their Best practices for writing Dockerfiles page, and it is used in an overwhelming number of blog posts and articles. We use it in many of our projects but not our base image.

This will be an easy change but needs to be done as new revisions of our images, not editing the existing dockerfile revisions.

[focusaurus]

• check documentation for places we might need to update

[aldeed]

Notes:

• also need to update path in entrypoint.sh
• projects that pull in this new base image version will need to update their docker-compose.dev.yml file volume paths
• search of other places that would need updating: https://github.com/search?p=1&q=org%3Areactioncommerce+%22%2Fusr%2Flocal%2Fsrc%2Fapp%22&type=Code