Allow expanding environment variables in image name

README.md

@@ -181,6 +181,14 @@ An array of additional files to pass into to the docker container as environment
 
 If you set this to `VALUE`, and `VALUE` is an environment variable containing a space-separated list of environment variables such as `A B C D`, then A, B, C, and D will all be propagated to the container. This is helpful when you've set up an `environment` hook to export secrets as environment variables, and you'd also like to programmatically ensure that secrets get propagated to containers, instead of listing them all out.
 
+### `expand-image-vars` (optional, boolean, unsafe)
+
+When set to true, it will activate interpolation of variables in the elements of the `image` configuration variable. When turned off (the default), attempting to use variables will fail as the literal `$VARIABLE_NAME` string will be passed as the image name.
+
+Environment variable interporation rules apply here. `$VARIABLE_NAME` is resolved at pipeline upload time, whereas `$$VARIABLE_NAME` is at run time. All things being equal, you likely want `$$VARIABLE_NAME`.
+
+:warning: **Important:** this is considered an unsafe option as the most compatible way to achieve this is to run the strings through `eval` which could lead to arbitrary code execution or information leaking if you don't have complete control of the pipeline
+
 ### `propagate-environment` (optional, boolean)
 
 Whether or not to automatically propagate all* pipeline environment variables into the docker container. Avoiding the need to be specified with `environment`.

hooks/command

@@ -365,10 +365,16 @@ if [[ "${BUILDKITE_PLUGIN_DOCKER_PROPAGATE_AWS_AUTH_TOKENS:-false}" =~ ^(true|on
   fi
 fi
 
+if [[ "${BUILDKITE_PLUGIN_DOCKER_EXPAND_IMAGE_VARS:-false}" =~ ^(true|on|1)$ ]] ; then
+  image=$(eval echo "${BUILDKITE_PLUGIN_DOCKER_IMAGE}")
+else
+  image="${BUILDKITE_PLUGIN_DOCKER_IMAGE}"
+fi
+
 if [[ "${BUILDKITE_PLUGIN_DOCKER_ALWAYS_PULL:-false}" =~ ^(true|on|1)$ ]] ; then
-  echo "--- :docker: Pulling ${BUILDKITE_PLUGIN_DOCKER_IMAGE}"
+  echo "--- :docker: Pulling ${image}"
   if ! retry "${BUILDKITE_PLUGIN_DOCKER_PULL_RETRIES:-3}" \
-       docker pull "${BUILDKITE_PLUGIN_DOCKER_IMAGE}" ; then
+       docker pull "${image}" ; then
     echo "!!! :docker: Pull failed."
     exit "$retry_exit_status"
   fi
@@ -487,7 +493,7 @@ fi
 args+=("--label" "com.buildkite.job-id=${BUILDKITE_JOB_ID}")
 
 # Add the image in before the shell and command
-args+=("${BUILDKITE_PLUGIN_DOCKER_IMAGE}")
+args+=("${image}")
 
 # Set a default shell if one is needed
 if [[ -z $shell_disabled ]] && [[ ${#shell[@]} -eq 0 ]] ; then
@@ -534,7 +540,7 @@ elif [[ ${#command[@]} -gt 0 ]] ; then
   done
 fi
 
-echo "--- :docker: Running command in ${BUILDKITE_PLUGIN_DOCKER_IMAGE}"
+echo "--- :docker: Running command in ${image}"
 echo -ne '\033[90m$\033[0m docker run ' >&2
 
 # Print all the arguments, with a space after, properly shell quoted

plugin.yml

@@ -25,6 +25,8 @@ configuration:
       type: array
     env-propagation-list:
       type: string
+    expand-image-vars:
+      type: boolean
     image:
       type: string
     ipc:

tests/command.bats

@@ -1242,3 +1242,36 @@ EOF
 
   unstub docker
 }
+
+@test "Do not expand image vars by default" {
+  export BUILDKITE_PLUGIN_DOCKER_IMAGE='123456789012.dkr.ecr.${AWS_DEFAULT_REGION}.amazonaws.com/image:tag'
+  export AWS_DEFAULT_REGION="us-east-1"
+  export BUILDKITE_COMMAND="pwd"
+
+  stub docker \
+    "run -t -i --rm --init --volume $PWD:/workdir --workdir /workdir --label com.buildkite.job-id=1-2-3-4 123456789012.dkr.ecr.\$\{AWS_DEFAULT_REGION\}.amazonaws.com/image:tag /bin/sh -e -c 'pwd' : echo ran command in docker"
+
+  run "$PWD"/hooks/command
+
+  assert_success
+  assert_output --partial "ran command in docker"
+
+  unstub docker
+}
+
+@test "Expand image vars" {
+  export BUILDKITE_PLUGIN_DOCKER_EXPAND_IMAGE_VARS=true
+  export BUILDKITE_PLUGIN_DOCKER_IMAGE='123456789012.dkr.ecr.${AWS_DEFAULT_REGION}.amazonaws.com/image:tag'
+  export AWS_DEFAULT_REGION="us-east-1"
+  export BUILDKITE_COMMAND="pwd"
+
+  stub docker \
+    "run -t -i --rm --init --volume $PWD:/workdir --workdir /workdir --label com.buildkite.job-id=1-2-3-4 123456789012.dkr.ecr.us-east-1.amazonaws.com/image:tag /bin/sh -e -c 'pwd' : echo ran command in docker"
+
+  run "$PWD"/hooks/command
+
+  assert_success
+  assert_output --partial "ran command in docker"
+
+  unstub docker
+}