Update CI actions using Docker script

When the config of the CI flow changes, we need to update the
`notify-workflow-completed`, `run-workflow` and `upstream-builds-query` actions
so that they would use the new config. This can be done separately for each
action (steps are described in the action's READMEs). Alternatively, this can be
done via the script I provide in this PR. The exact steps are described in the
comments in the script.

actions/Dockerfile

@@ -0,0 +1,49 @@
+# This Dockerfile can be used to quickly generate the needed artifacts when
+# updating `notify-workflow-completed`, `run-workflow` and `upstream-builds-query`
+# custom GH actions after changes in the `./config/config.json` file.
+# To update the artifacts:
+# 1. Push the modified `./config/config.json` to a feature branch.
+# 2. Provide the name of the feature branch to the `BRANCH_NAME` parameter below.
+# 3. Build the Docker image:
+#      ```
+#      docker build --tag actions .
+#      ```
+# 4. Run the Docker image (the container will be created and will run for 5 min):
+#      ```
+#      docker run -it actions
+#      ```
+# 5. List the Docker images and copy the container id:
+#      ```
+#      docker ps
+#      ```
+# 6. Replace the container id in the following commands and run them from the root of the `ci` project:
+#      ```
+#      docker cp 66c205e35635:/workdir/ci/actions/notify-workflow-completed/dist/ ./notify-workflow-completed
+#      docker cp 66c205e35635:/workdir/ci/actions/run-workflow/dist/ ./run-workflow             
+#      docker cp 66c205e35635:/workdir/ci/actions/upstream-builds-query/dist/ ./upstream-builds-query
+#      ```
+# 7. Commit the changes in the actions to your feature branch.
+
+FROM node:14-alpine3.13 AS build
+
+ENV BRANCH_NAME=feature_branch_with_the_new_config
+
+RUN apk add --update --no-cache \
+  git \
+  bash
+
+WORKDIR /workdir
+
+RUN git clone https://github.com/keep-network/ci -b $BRANCH_NAME
+
+WORKDIR ci/actions/notify-workflow-completed
+RUN yarn install
+RUN yarn run prepare
+WORKDIR ../run-workflow
+RUN yarn install
+RUN yarn run prepare
+WORKDIR ../upstream-builds-query
+RUN yarn install
+RUN yarn run prepare
+
+CMD sleep 300