docker: include git hash in Docker image.

Put the Git hash of the current commit inside the Docker image built by
Docker Hub. The hash is stored in the REVISION file in the root directory.

config/docker/Dockerfile.danbooru

@@ -47,6 +47,9 @@ RUN yarn install
 
 COPY . .
 
+ARG SOURCE_COMMIT
+RUN echo "$SOURCE_COMMIT" > REVISION
+
 
 
 FROM development AS assets

hooks/README.md

@@ -0,0 +1,27 @@
+The directory contains hooks used by Docker Hub when building images:
+
+    Docker Hub allows you to override and customize the `build`, `test` and `push`
+    commands during automated build and test processes using hooks. For
+    example, you might use a build hook to set build arguments used only during
+    the build process. (You can also set up custom build phase hooks to perform
+    actions in between these commands.)
+
+    Use these hooks with caution. The contents of these hook files replace the
+    basic `docker` commands, so you must include a similar build, test or push
+    command in the hook or your automated process does not complete.
+
+    To override these phases, create a folder called `hooks` in your source code
+    repository at the same directory level as your Dockerfile. Create a file
+    called `hooks/build`, `hooks/test`, or `hooks/push` and include commands that the
+    builder process can execute, such as `docker` and `bash` commands (prefixed
+    appropriately with `#!/bin/bash`).
+
+    These hooks will be running on an instance of Amazon Linux 2, a distro
+    based on Ubuntu, which includes interpreters such as Perl and Python and
+    utilities such as git or curl. Please check the link above for the full
+    list.
+
+# See also
+
+* https://docs.docker.com/docker-hub/builds/advanced
+* https://stackoverflow.com/questions/59057978/passing-source-commit-to-dockerfile-commands-on-docker-hub

hooks/build

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+# This file is used to add the Git commit hash to the Docker image when the
+# image is built by Docker Hub.
+#
+# https://docs.docker.com/docker-hub/builds/advanced/
+# https://stackoverflow.com/questions/59057978/passing-source-commit-to-dockerfile-commands-on-docker-hub
+#
+# SOURCE_COMMIT: the SHA1 hash of the commit being tested.
+# DOCKERFILE_PATH: the dockerfile currently being built.
+# IMAGE_NAME: the name and tag of the Docker repository being built. (This variable is a combination of DOCKER_REPO:DOCKER_TAG.)
+
+docker build --build-arg SOURCE_COMMIT=$SOURCE_COMMIT -f $DOCKERFILE_PATH -t $IMAGE_NAME .