replace CACHE_BUST with DEXBUILD_NOCACHE

Author: briceburg

docs/HOWTO.md

@@ -151,24 +151,29 @@ dex run --pull local/my-app
 ```
 
 
-#### busting cache
+#### busting cache ( DEXBUILD_NOCACHE )
 
-Often, images will use a git repository to install an application. E.g.
+Sometimes images will use a git repository to install an application. E.g.
 
 ```
 # ...
 RUN git clone my-repo/my-app.git /app
 # ...
 ```
 
-Docker will cache this, and use it's cache for subsequent builds -- no matter
-if the git repository and application code has been changed upstream. To get
-around this, dex builds images with a CACHE_BUST argument. Use this to introduce
-randomness and force the git clone command. E.g.
+The command is fingerprinted and its results cached in Docker's build-cache.
+On subsequent builds the command fingerprint maintains the same, so Docker
+returns the results from the build-cache. Your application code WILL NOT CHANGE
+no matter if changes have been made upstream.
+
+Dex provides a convenient way around this cache. Invoke the `DEXBUILD_NOCACHE`
+build argument and _all_ subsequent commands will have a changed fingerprint --
+and thus execute.
 
 ```
 # ...
-ARG CACHE_BUST
+# bust the cache so git clone (and ALL subsequent commands) runs
+ARG DEXBUILD_NOCACHE
 RUN git clone my-repo/my-app.git /app
 # ...
 ```

lib.d/dex-image.sh

@@ -43,8 +43,8 @@ dex-image-build(){
         cd $image_dir
 
         # add cachebusting argument if requested/used in Dockerfile
-        grep -q "^ARG CACHE_BUST" $dockerfile &&  \
-          cachebust="--build-arg CACHE_BUST=$random"
+        grep -q "^ARG DEXBUILD_NOCACHE" $dockerfile &&  \
+          cachebust="--build-arg DEXBUILD_NOCACHE=$random"
 
         $__pull_flag && \
           pull="--pull"

tests/bats/05-image.bats

@@ -89,7 +89,7 @@ setup(){
   [ "$first_sha" = "$second_sha" ]
 }
 
-@test "image build uses CACHE_BUST argument to circumvent docker build cache" {
+@test "image build uses DEXBUILD_NOCACHE argument to circumvent docker build cache" {
   run $DEX image build imgtest/cachebust:nocache
   [ $status -eq 0 ]
   first_sha=$(docker inspect -f '{{ .Id }}' $DEX_NAMESPACE/cachebust:nocache)

tests/fixtures/dex-images/cachebust/Dockerfile-nocache

@@ -4,5 +4,5 @@ LABEL org.dockerland.dex.api="v1"
 CMD echo "ALPINE_RELEASE=$(cat /etc/alpine-release)" ; printenv
 
 # bust some cache
-ARG CACHE_BUST
+ARG DEXBUILD_NOCACHE
 RUN echo "I always execute"