Add Dockerfile for application and documentation

This commit adds a `Dockerfile` to the root of the project, accompanied
by a `docker-compose.yml.example` for simplified deployment. The
`Dockerfile` is agnostic to whether it will be the webserver, the
consumer, or if it is run for a one-off command (i.e. creation of a
superuser, migration of the database, document export, ...).

The containers entrypoint is the `scripts/docker-entrypoint.sh` script.
This script verifies that the required permissions are set, remaps the
default users and/or groups id if required and installs additional
languages if the user wishes to.

After initialization, it analyzes the command the user supplied:

  - If the command starts with a slash, it is expected that the user
    wants to execute a binary file and the command will be executed
    without further intervention. (Using `exec` to effectively replace
    the started shell-script and not have any reaping-issues.)

  - If the command does not start with a slash, the command will be
    passed directly to the `manage.py` script without further
    modification. (Again using `exec`.)

The default command is set to `--help`.

If the user wants to execute a command that is not meant for `manage.py`
but doesn't start with a slash, the Docker `--entrypoint` parameter can
be used to circumvent the mechanics of `docker-entrypoint.sh`.

Further information can be found in `docs/setup.rst` and in
`docs/migrating.rst`.

For additional convenience, a `Dockerfile` has been added to the `docs/`
directory which allows for easy building and serving of the
documentation. This is documented in `docs/requirements.rst`.

scripts/docker-entrypoint.sh

@@ -0,0 +1,74 @@
+#!/bin/bash
+set -e
+
+# Source: https://github.com/sameersbn/docker-gitlab/
+map_uidgid() {
+    USERMAP_ORIG_UID=$(id -u paperless)
+    USERMAP_ORIG_UID=$(id -g paperless)
+    USERMAP_GID=${USERMAP_GID:-${USERMAP_UID:-$USERMAP_ORIG_GID}}
+    USERMAP_UID=${USERMAP_UID:-$USERMAP_ORIG_UID}
+    if [[ ${USERMAP_UID} != ${USERMAP_ORIG_UID} || ${USERMAP_GID} != ${USERMAP_ORIG_GID} ]]; then
+        echo "Mapping UID and GID for paperless:paperless to $USERMAP_UID:$USERMAP_GID"
+        groupmod -g ${USERMAP_GID} paperless
+        sed -i -e "s|:${USERMAP_ORIG_UID}:${USERMAP_GID}:|:${USERMAP_UID}:${USERMAP_GID}:|" /etc/passwd
+    fi
+}
+
+set_permissions() {
+    # Set permissions for consumption directory
+    chgrp paperless "$PAPERLESS_CONSUME"
+    chmod g+x "$PAPERLESS_CONSUME"
+
+    # Set permissions for application directory
+    chown -Rh paperless:paperless /usr/src/paperless
+}
+
+initialize() {
+    map_uidgid
+    set_permissions
+}
+
+install_languages() {
+    local langs="$1"
+    read -ra langs <<<"$langs"
+
+    # Check that it is not empty
+    if [ ${#langs[@]} -eq 0 ]; then
+        return
+    fi
+
+    # Update apt-lists
+    apt-get update
+
+    # Loop over languages to be installed
+    for lang in "${langs[@]}"; do
+        pkg="tesseract-ocr-$lang"
+        if dpkg -s "$pkg" 2>&1 > /dev/null; then
+            continue
+        fi
+
+        if ! apt-cache show "$pkg" 2>&1 > /dev/null; then
+            continue
+        fi
+
+        apt-get install "$pkg"
+    done
+
+    # Remove apt lists
+    rm -rf /var/lib/apt/lists/*
+}
+
+
+if [[ "$1" != "/"* ]]; then
+    initialize
+
+    # Install additional languages if specified
+    if [ ! -z "$PAPERLESS_OCR_LANGUAGES"  ]; then
+        install_languages "$PAPERLESS_OCR_LANGUAGES"
+    fi
+
+    exec sudo -HEu paperless "/usr/src/paperless/src/manage.py" "$@"
+fi
+
+exec "$@"
+