Fix for skipping push to dockerhub from travis jobs on PRs (#875)

Correctly skip builds on forks by checking the value of TRAVIS_PULL_REQUEST variable.

.popper.yml

@@ -1,12 +1,15 @@
 steps:
-- id: skip if not in master branch, docker, and python3.7
+- id: skip on forks
   uses: docker://docker:19.03.3
-  runs: [sh, -ec]
-  secrets: [ENGINE, TRAVIS_PYTHON_VERSION]
+  runs: [sh, -exc]
+  secrets: [ENGINE, TRAVIS_PYTHON_VERSION, TRAVIS_PULL_REQUEST]
   args:
   - |
-    # only run on master, docker and python 3.7
-    if [ "$GIT_BRANCH" != "master" ] || [ "$ENGINE" != "docker" ] || [ "$TRAVIS_PYTHON_VERSION" != "3.7" ] ; then
+    # skip if this is a build for a PR
+    # for the rest, only continue if build:
+    #   1. is for upstream (getpopper/popper) repo
+    #   2. corresponds to the docker,python-3.7 element of the matrix
+    if [[ $TRAVIS_PULL_REQUEST != false ]] || [[ $GIT_REMOTE_ORIGIN_URL != "https://github.com/getpopper/popper.git" ]] || [[ $GIT_BRANCH != "master" ]] || [[ $ENGINE != "docker" ]] || [[ $TRAVIS_PYTHON_VERSION != "3.7" ]]; then
       exit 78
     fi
 

install.sh

@@ -16,6 +16,7 @@ cat > ./popper << "EOF"
 printenv > /tmp/.envfile
 
 docker run --rm -ti \
+  --volume /tmp:/tmp \
   --volume /var/run/docker.sock:/var/run/docker.sock \
   --volume $PWD:$PWD \
   --workdir $PWD \