Added additional descriptions about the version envsubst

Signed-off-by: James Ketrenos <james.p.ketrenos@intel.com>

README.md

@@ -37,6 +37,16 @@ is changed to:
 
   LIBGL1_MESA_GLX_VERSION=19.0.1-agama-109
 
+This allows the Dockerfile templates to then version pin Agama packages:
+
+```Dockerfile
+  RUN apt-get install -y libgl1-mesa-glx=$LIBGL1_MESA_GLX_VERSION
+```
+
+The `scripts/build-dockerfile` loads MANIFEST, which defines
+LIBGL1_MESA_GLX_VERSION. That is then subsituted for the version in
+the above Dockerfile snippet when being placed into the main Dockerfile.
+
 The generic script I ran to generate these names is below, although
 it doesn't quite work and a few entries were modified by hand. This should
 be replaced by a tool that is actually querying the .deb packages and 
@@ -53,3 +63,13 @@ find /var/www/packages/ -path '*public*' -name '*deb' -printf '%f\n' |
     -e s,-\([^0-9]\),_\\1,g \
     -e 's,^([^=]*),\U\1,'
 ```
+
+# Tagging
+
+If the build succeeds, we want to be able to tag the git project
+as well as corresponding Docker images with the appropriate Agama
+tag:
+
+```bash
+. MANIFEST ; git tag -f agama-${AGAMA_VERSION}
+```

scripts/build-dockerfile

@@ -29,4 +29,14 @@ EOM
 
 envsubst < Dockerfile.solution >> Dockerfile
 
-docker build . -t $CONTAINER
+docker build . -t $CONTAINER:agama-${AGAMA_VERSION}
+
+cat << EOM
+
+If the build looks good, you should commit the changes
+to Dockerfile and tag it as agama-${AGAMA_VERSION}:
+
+  git commit -s -a -m "Build of agama-${AGAMA_VERSION}"
+  git tag -f agama-${AGAMA_VERSION}
+
+EOM