Equivalent builds don't behave the same

if I build an image like this:

docker build -f app/Dockerfile -t test .

it builds just fine, but the following docker-compose.yml (which I believe should be equivalent):

app:
  container_name: test
  build:
    context: .
    dockerfile: app/Dockerfile

fails to build. if I run docker-compose with --verbose I see:

compose.cli.verbose_proxy.proxy_callable: docker build <- (pull=False, stream=True, nocache=False, tag=u'prj_app', buildargs=None, rm=True, forcerm=False, path='/Users/ekkis/dev/prj', dockerfile='./app/Dockerfile')

which would seem pretty much what I expect. specifically the error I'm getting is this (sorry to include as code but quotes make a mess of things):

npm WARN app@1.0.0 No description  
npm WARN app@1.0.0 No repository field.  
npm ERR! Linux 4.9.12-moby 
npm ERR! argv "/usr/local/bin/node" "/usr/local/bin/npm" "install" "/tmp/inc" 
npm ERR! node v7.7.3 
npm ERR! npm  v4.1.2 
npm ERR! path /usr/src/app/node_modules/inc 
npm ERR! code ENOENT 
npm ERR! errno -2 
npm ERR! syscall access
npm ERR! enoent ENOENT: no such file or directory, access '/usr/src/app/node_modules/inc' 
npm ERR! enoent ENOENT: no such file or directory, access '/usr/src/app/node_modules/inc' 
npm ERR! enoent This is most likely not a problem with npm itself 
npm ERR! enoent and is related to npm not being able to find a file. 
npm ERR! enoent 
npm ERR! Please include the following file with any support request: 
npm ERR!     /usr/src/app/npm-debug.log

as you can see from the above, the Dockerfile performs an npm install /tmp/inc, which from the output I see succeeded since it prints a tree of modules installed. /tmp/inc is created by the Dockerfile command shown below as successful:

Step 8/11 : ADD inc /tmp/inc
 ---> Using cache
 ---> fce607bb13ca

and I note that the app/npm-debug.log was not written to, nor is there a log file in the directory where I ran docker compose.

what could be the issue here? why does it build ok manually but not with docker-compose?

* Addendum I *

in my continuing efforts to figure it out, I added a directory listing before the Dockerfile executes the npm install...

Step 9/13 : RUN ls -alF
 ---> Running in ac0c62b7da93
total 28
drwxr-xr-x   3 root root 4096 Mar 19 07:01 ./
drwxr-xr-x   4 root root 4096 Mar 19 07:01 ../
-rw-r--r--   1 root root  722 Mar 19 05:48 config.json
-rw-r--r--   1 root root 3786 Mar 19 05:48 index.js
drwxr-xr-x 119 root root 4096 Mar 17 18:57 node_modules/
-rw-r--r--   1 root root 2910 Mar 17 20:51 npm-debug.log
-rw-r--r--   1 root root  416 Mar 19 06:39 package.json

...and, most interestingly, there is already a node_modules directory there!! that means the problem is that docker-compose is not honouring the .dockerignore file (which prevents copying **/node_modules into the context)

so is there some special voodoo I have to perform for the build to generate the context properly?

via ekkis