[docker] Reduce the number of build steps in containers

[docker] Reduce the number of build steps in containers

* Move collectd_apply_pull_request.sh to src/collectd/, which is
  where the Makefile expects this script to be anyway

* Use ARG instead of ENV in collectd container
  for COLLECTD_FLAVOR, COLLECTD_PULL_REQUESTS, WITH_DPDK

* Move the experimental sample configs to src/collectd
  The configs are copied into the src/collectd directory within the
  container anyway, so this skips a step in the container build.

* Remove initial package installation in containers
  Move the installation of these packages to the package install steps
  in the build script in systems/ instead.

* Remove unnecessary WORKDIR command
  The WORKDIR {repo_dir} is unnecessary, since the following commands
  are using absolute paths rather than relative paths.

Signed-off-by: Emma Foley <efoley@redhat.com>
Change-Id: I52b96a651cb5d9b644f11f5407cabf3fb21cb6a9