docker: support including plugins in custom builds.

This commit introduces the support for the ETHERPAD_PLUGINS build parameter,
which contains a list of plugins to be installed while building the container.

EXAMPLE:
  docker build --build-arg ETHERPAD_PLUGINS="ep_codepad ep_author_neat" --tag <YOUR_USERNAME>/etherpad .

Resolves #3618.

docker/Dockerfile

@@ -13,6 +13,14 @@ LABEL maintainer="Etherpad team, https://github.com/ether/etherpad-lite"
 # If not given, build the latest development version.
 ARG ETHERPAD_VERSION=develop
 
+# plugins to install while building the container. By default no plugins are
+# installed.
+# If given a value, it has to be a space-separated, quoted list of plugin names.
+#
+# EXAMPLE:
+#   ETHERPAD_PLUGINS="ep_codepad ep_author_neat"
+ARG ETHERPAD_PLUGINS=
+
 # Set the following to production to avoid installing devDeps
 # this can be done with build args (and is mandatory to build ARM version)
 ARG NODE_ENV=development
@@ -38,6 +46,12 @@ WORKDIR /opt/etherpad-lite
 # install node dependencies for Etherpad
 RUN bin/installDeps.sh
 
+# Install the plugins, if ETHERPAD_PLUGINS is not empty.
+#
+# Bash trick: in the for loop ${ETHERPAD_PLUGINS} is NOT quoted, in order to be
+# able to split at spaces.
+RUN for PLUGIN_NAME in ${ETHERPAD_PLUGINS}; do npm install "${PLUGIN_NAME}"; done
+
 # Copy the custom configuration file, if present. The configuration file has to
 # be manually put inside the same directory containing the Dockerfile (we cannot
 # directly point to "../settings.json" for Docker's security restrictions).

docker/README.md

@@ -27,6 +27,12 @@ cp ../settings.json.template settings.json
 
 **Each configuration parameter can also be set via an environment variable**, using the syntax `"${ENV_VAR}"` or `"${ENV_VAR:default_value}"`. For details, refer to `settings.json.template`.
 
+## Rebuilding including some plugins
+If you want to install some plugins in your container, it is sufficient to list them in the ETHERPAD_PLUGINS build variable.
+The variable value has to be a space separated, double quoted list of plugin names (see examples).
+
+Some plugins will need personalized settings in the `settings.json` file. Just refer to the previous section, and include them in your custom `settings.json`.
+
 ## Examples
 
 Build the latest development version:
@@ -49,6 +55,11 @@ Build a specific git hash:
 docker build --build-arg ETHERPAD_VERSION=4c45ac3cb1ae --tag <YOUR_USERNAME>/etherpad .
 ```
 
+Include two plugins in the container:
+```bash
+docker build --build-arg ETHERPAD_PLUGINS="ep_codepad ep_author_neat" --tag <YOUR_USERNAME>/etherpad .
+```
+
 # Running your instance:
 
 To run your instance: