Moving the long RUN command into a shell script

Adding support for catching the command failure and restoring the permissions of the build dir so go can still Clean up the directory.
2. Adding support for pre_package hook that runs before the slug tarball is generated

Dockerfile

@@ -16,14 +16,10 @@ RUN curl --silent http://dl.gliderlabs.com/herokuish/latest/linux_x86_64.tgz \
     | tar -xzC /bin
 
 COPY mesos-fix /bin/mesos-fix
+COPY build-slug /bin/build-slug
 
 RUN /bin/herokuish buildpack install  \
-    && chmod +x /bin/mesos-fix
-
-CMD bash -c "addgroup --gid $GROUP_ID go \
-    && adduser -q --disabled-password --gid $GROUP_ID --uid $USER_ID --gecos \"\" --shell /bin/bash --home /var/go go \
-    && (/bin/herokuish buildpack build || chown -R go:go /app && false) \
-    && /bin/mesos-fix \
-    && /bin/herokuish slug generate \
-    && /bin/herokuish slug export > /app/app.tar.gz \
-    && chown -R go:go /app"
+    && chmod +x /bin/mesos-fix  \
+    && chmod +x /bin/build-slug
+
+CMD /bin/build-slug

README.md

@@ -10,5 +10,8 @@ We expect the app material to be mounted on `/app` inside the container. After l
 $ docker run -e USER_ID="$(id -u $(whoami))" -e GROUP_ID="$(id -g $(whoami))" -v $PWD/app:/app ind9/python-stack
 ```
 
+## Hooks
+- `bin/pre_package` - Run before the generating the slug tarball. Any changes to the directory structure will be reflected in the generated slug.
+
 ### Note
 We need to do that `id` foo while doing `docker run` because else the mounted directory would have root files which GoCD can't delete or overwrite on the next run.

build-slug

@@ -0,0 +1,26 @@
+#!/bin/bash
+
+set -x -e
+
+run_in_sandbox() {
+  CMD=$1
+  bash -c "${CMD} || (chown -R go:go /app && false)"
+}
+
+addgroup --gid $GROUP_ID go
+adduser -q --disabled-password --gid $GROUP_ID --uid $USER_ID --gecos \"\" --shell /bin/bash --home /var/go go
+run_in_sandbox "/bin/herokuish buildpack build"
+run_in_sandbox /bin/mesos-fix
+
+# Experimental - bin/pre_package hook run before the generating the slug tarball
+pushd /app
+if [ -f bin/pre_package ]; then
+    echo "-----> Running pre_package hook"
+    chmod +x bin/pre_package
+    ./bin/pre_package
+fi
+popd
+
+run_in_sandbox "/bin/herokuish slug generate"
+run_in_sandbox "/bin/herokuish slug export > /app/app.tar.gz"
+chown -R go:go /app