WIP: add migration script and docs

docs/migrate-repo-and-db.md

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+# Migration without script
+## Checking disk space
+First you should check the available disk space with commands like `df` and `du`. You
+should have at least two times the size of your *volumes/db* folder in free space as we need to copy it.
+
+```
+cd mattermost-docker
+sudo df -h
+sudo du -sh volumes/db
+```
+
+If your installation resides on a Btrfs with snapshots it's advisable to run the following commands:
+```
+sudo btrfs filesystem df -h volumes
+sudo btrfs filesystem du -s volumes/db
+```
+
+## Backup database
+For a database backup you can use two different methods.
+```
+cd mattermost-docker
+mkdir backup
+mv $PWD/{.circleci,.git,.gitignore,.travis.yml,app,contrib,db,web,CONTRIBUTING.md,LICENSE,MAINTAINANCE.md,README.md,docker-compose.yml} backup
+
+git init
+git remote add origin https://github.com/mattermost/docker
+git pull origin main
+```

scripts/migrate-repo-and-db.sh

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+#!/bin/bash
+
+set -x
+
+usage() {
+  cat <<EOF
+Usage: $0 [-h]
+
+Options
+  -h Print this help
+  -d Path to database folder which contains database data (default: ./volumes/db/var/lib/postgresql/data)
+  -e Path to env-file (default: ./.env)
+
+EOF
+}
+
+pre_checks() {
+  if ! eval which docker &>/dev/null; then
+    echo "Can't find 'docker' command. Is the Docker installed an running?" >&2
+    exit 64
+  fi
+
+  if ! eval which docker-compose &>/dev/null; then
+    echo "Can't find 'docker-compose' command. Is it installed and in your $PATH?" >&2
+    exit 64
+  fi
+
+  if ! eval which git &>/dev/null; then
+    echo "Can't find 'git' command. Is it installed and in your $PATH?" >&2
+    exit 64
+  fi
+}
+
+become_root() {
+  # become root while keeping the environment and make script executable
+  if [[ $EUID != 0 ]]; then
+    chmod +x "$0"
+    sudo -E ./"$0" "$@"
+    exit $?
+  fi
+}
+
+info() { printf "\e[1m%s\e[0m\n" "$*" >&2; }
+
+print_warning() {
+  echo
+  printf "%s\n" "$@" >&2
+}
+
+print_error() {
+  echo
+  printf "%s\n" "$@" >&2
+  exit 1
+}
+
+choice() {
+  while true; do
+    echo -n "$1" "(y/N) "
+    read -r decision
+    if [[ "$decision" =~ (y|Y|j|J) ]]; then
+      return 1
+    elif [[ "$decision" =~ (n|N) ]]; then
+      return 0
+    fi
+  done
+}
+
+check_disk_space() {
+  DB_PATH="$1"
+  FACTOR="$2"
+
+  info "Checking if enough disk space is available."
+  AVAIL=$(df -k --output=avail "$PWD" | tail -n1)
+  DB_SIZE=$(du -k -s "$DB_PATH" | awk '{print $1}')
+
+  if [[ "$(stat -f --format="%T" $REPO_PATH)" != 'btrfs' ]]; then
+    if [[ "$AVAIL" -gt $(("$DB_SIZE" * "$FACTOR")) ]]; then
+      return 0
+    else
+      return 1
+    fi
+  else
+    print_error "Btrfs is not supported by this script yet. Please check the available" \
+                "disk space with 'df -h' and 'du -sh PATH-TO-DB-VOLUME'. To skip the" \
+                "check please execute the script with '-s' to continue."
+  fi
+}
+
+move_old_repo() {
+  if [[ ! -f "$REPO_PATH/MOVED" ]]; then
+    info "Moving old mattermost-docker repo to backup folder"
+    mkdir "$REPO_PATH"/backup
+    find "$REPO_PATH" -maxdepth 1 -mindepth 1 \
+      -not -name volumes \
+      -not -name backup \
+      -not -name upgrade-postgres.sh \
+      -not -name MOVED \
+      -not -name INIT \
+      -print0 | xargs -0 mv -t "$REPO_PATH"/backup
+    touch "$REPO_PATH/MOVED"
+  fi
+}
+
+init_new_repo() {
+  if [[ ! -f "$REPO_PATH/INIT" ]]; then
+    info "Initilizing new docker repo"
+    pushd "$REPO_PATH" >/dev/null
+
+    git init
+    git remote add origin https://github.com/mattermost/docker
+    git pull origin main 1> /dev/null
+
+    popd >/dev/null
+
+    touch "$REPO_PATH/INIT"
+  fi
+
+  print_warning "Please open another terminal and copy the 'env.example' to '.env'"
+                "and edit this '.env' to match your previous settings."
+  while ! choice "Finished editing '.env'? Can we proceed?"; do
+    continue
+  done
+}
+
+backup_database() {
+  info "Database backup"
+  if choice "Do you want to backup the database with 'pg_dumpall'? This spins up your database temporarily"; then
+    mkdir -p "$REPO_PATH"/backup/volumes/db
+
+    info "Attempting to backup database with 'pg_dumpall'"
+    #-v $POSTGRES_DATA_PATH:/var/lib/postgresql/data:Z \
+    docker run -d --rm --name mattermost-postgres \
+      -e POSTGRES_USER="$POSTGRES_USER" \
+      -e POSTGRES_PASSWORD="$POSTGRES_PASSWORD" \
+      -e POSTGRES_DB="$POSTGRES_DB" \
+      -v $REPO_PATH/volumes/db/var/lib/postgresql/data:/var/lib/postgresql/data:Z \
+      postgres:9.4-alpine
+
+    docker exec mattermost-postgres /bin/bash -c "pg_dumpall -U \"$POSTGRES_USER\" > /var/lib/postgresql/data/BACKUP_DUMP.sql"
+    docker stop -t 5 mattermost-postgres
+    mv "$POSTGRES_DATA_PATH/BACKUP_DUMP.sql" "$REPO_PATH"/backup/volumes/db/
+
+    print_warning "Dump written to POSTGRES_DATA_PATH/BACKUP_DUMP.sql (inside container). Moved it to backup/volumes/db."
+  fi
+
+  if choice "Do you want to backup the database on filesystem level?"; then
+    info "Attempting to backup database with 'cp'..."
+    # keeping original structure
+    mkdir -p $REPO_PATH/backup/volumes/db/var/lib/postgresql
+    cp -a "$POSTGRES_DATA_PATH" "$REPO_PATH"/backup/volumes/db/var/lib/postgresql/
+
+    print_warning "$POSTGRES_DATA_PATH copied into $REPO_PATH/backup/volumes/db/var/lib/postgresql/."
+  fi
+}
+
+migrate_database() {
+  info "Starting migration to Postgres 13."
+  mv "$REPO_PATH"/volumes/db/var/lib/postgresql/data "$REPO_PATH"/volumes/db/var/lib/postgresql/data-old
+  mkdir "$REPO_PATH"/volumes/db/var/lib/postgresql/data
+
+  # the postgres process is running with user id 999 inside the container
+  # to date the container changes them itself
+  #chown -R 999:999 "$REPO_PATH"/volumes/db/var/lib/postgresql
+
+  docker run --rm --name postgres-upgrade \
+    -e POSTGRES_INITDB_ARGS=" -U $POSTGRES_USER" \
+    -e PGUSER="$POSTGRES_USER" \
+    -e POSTGRES_USER="$POSTGRES_USER" \
+    -e POSTGRES_PASSWORD="$POSTGRES_PASSWORD" \
+    -e POSTGRES_DB="$POSTGRES_DB" \
+    -v "$REPO_PATH"/volumes/db/var/lib/postgresql/data-old:/var/lib/postgresql/9.4/data:Z \
+    -v "$REPO_PATH"/volumes/db/var/lib/postgresql/data:/var/lib/postgresql/13/data:Z \
+    tianon/postgres-upgrade:9.4-to-13
+}
+
+#trap '{ rm -rf "$REPO_PATH/MOVED" "$REPO_PATH/INIT"; }' 0
+
+become_root
+
+while getopts srh opt; do
+  case "$opt" in
+    s)
+      skip_check=$OPTARG
+      ;;
+    r)
+      repo_path=$OPTARG
+      ;;
+    h)
+      usage
+      exit 0
+      ;;
+    \?)
+      usage >&2
+      exit 64
+      ;;
+  esac
+done
+
+shift $((OPTIND - 1))
+
+pre_checks
+move_old_repo
+init_new_repo
+# read in values from env file
+. "${REPO_PATH}/.env"
+backup_database
+migrate_database