Dramatically simplify image_build role

This does a few things:

- Removes need for awx_sdist_builder image
- Reorders Dockerfile steps to optimize image cache between prod and dev builds
- Unifies VENV_BASE and COLLECTION_BASE in prod and dev builds

installer/roles/image_build/files/Dockerfile.sdist

@@ -1,22 +0,0 @@
-FROM centos:8
-
-RUN dnf -y update && dnf -y install epel-release && \
-    dnf install -y bzip2 \
-    gcc-c++ \
-    gettext \
-    git \
-    make \
-    nodejs \
-    python3 \
-    python3-setuptools
-
-# Use the distro provided npm to bootstrap our required version of node
-RUN npm install -g n && n 14.15.1 && dnf remove -y nodejs
-
-RUN mkdir -p /.npm && chmod g+rwx /.npm
-
-ENV PATH=/usr/local/n/versions/node/14.15.1/bin:$PATH
-
-WORKDIR "/awx"
-
-CMD ["make", "sdist"]