Bugfix: docker-compose instructions create a sonic container that fails to start
#### Describe the bug

I followed the docker-compose instructions from the README. This is the result:

```
[root@Acheron archivebox]# docker-compose ps
         Name                        Command                State             Ports
--------------------------------------------------------------------------------------------
archivebox_archivebox_1   dumb-init -- /app/bin/dock ...   Up         0.0.0.0:8000->8000/tcp
archivebox_sonic_1        sonic -c /etc/sonic.cfg          Exit 101
```
```
[root@Acheron archivebox]# docker-compose logs sonic
Attaching to archivebox_sonic_1
sonic_1       | thread 'main' panicked at 'cannot read config file: Os { code: 21, kind: Other, message: "Is a directory" }', src/config/reader.rs:24:14
sonic_1       | note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
sonic_1       | thread 'main' panicked at 'cannot read config file: Os { code: 21, kind: Other, message: "Is a directory" }', src/config/reader.rs:24:14
sonic_1       | note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

Search seems to work anyway.

I would expect one of:

a. `sonic` container is not created by default if it requires the user to manually create a config and is not necessary to run ArchiveBox
b. `config.cfg` is created for me by the init script, using the environment variable I set in the docker-compose file
c. `config.cfg` is not required by sonic (however, this is not the case: https://github.com/valeriansaliou/sonic/issues/197)

#### Steps to reproduce

From the README:

```
# create a new empty directory and initalize your collection (can be anywhere)
mkdir ~/archivebox && cd ~/archivebox
curl -O https://raw.githubusercontent.com/ArchiveBox/ArchiveBox/master/docker-compose.yml
docker-compose run archivebox init
docker-compose run archivebox --version

# start the webserver and open the UI (optional)
docker-compose run archivebox manage createsuperuser
docker-compose up -d
open http://127.0.0.1:8000

# you can also add links and manage your archive via the CLI:
docker-compose run archivebox add 'https://example.com'
docker-compose run archivebox status
docker-compose run archivebox help  # to see more options
```

#### ArchiveBox version
```
[root@Acheron archivebox]# docker-compose run archivebox --version
Starting archivebox_sonic_1 ... done
Creating archivebox_archivebox_run ... done
ArchiveBox v0.5.3
Cpython Linux Linux-5.9.1-arch1-1-x86_64-with-glibc2.28 x86_64 (in Docker)

[i] Dependency versions:
 √  ARCHIVEBOX_BINARY     v0.5.3          valid     /usr/local/bin/archivebox
 √  PYTHON_BINARY         v3.9.1          valid     /usr/local/bin/python3.9
 √  DJANGO_BINARY         v3.1.3          valid     /usr/local/lib/python3.9/site-packages/django/bin/django-admin.py
 √  CURL_BINARY           v7.64.0         valid     /usr/bin/curl
 √  WGET_BINARY           v1.20.1         valid     /usr/bin/wget
 √  NODE_BINARY           v15.5.1         valid     /usr/bin/node
 √  SINGLEFILE_BINARY     v0.1.14         valid     /node/node_modules/single-file/cli/single-file
 √  READABILITY_BINARY    v0.1.0          valid     /node/node_modules/readability-extractor/readability-extractor
 √  MERCURY_BINARY        v1.0.0          valid     /node/node_modules/@postlight/mercury-parser/cli.js
 √  GIT_BINARY            v2.20.1         valid     /usr/bin/git
 √  YOUTUBEDL_BINARY      v2021.01.03     valid     /usr/local/bin/youtube-dl
 √  CHROME_BINARY         v87.0.4280.88   valid     /usr/bin/chromium
 √  RIPGREP_BINARY        v0.10.0         valid     /usr/bin/rg

[i] Source-code locations:
 √  PACKAGE_DIR           22 files        valid     /app/archivebox
 √  TEMPLATES_DIR         3 files         valid     /app/archivebox/themes

[i] Secrets locations:
 -  CHROME_USER_DATA_DIR  -               disabled
 -  COOKIES_FILE          -               disabled

[i] Data locations:
 √  OUTPUT_DIR            6 files         valid     /data
 √  SOURCES_DIR           1 files         valid     ./sources
 √  LOGS_DIR              0 files         valid     ./logs
 √  ARCHIVE_DIR           1 files         valid     ./archive
 √  CONFIG_FILE           81.0 Bytes      valid     ./ArchiveBox.conf
 √  SQL_INDEX             204.0 KB        valid     ./index.sqlite3
```

```
[root@Acheron archivebox]# docker version
Client:
 Version:           20.10.2
 API version:       1.40
 Go version:        go1.15.6
 Git commit:        2291f610ae
 Built:             Tue Jan  5 19:56:21 2021
 OS/Arch:           linux/amd64
 Context:           default
 Experimental:      true

Server:
 Engine:
  Version:          19.03.13-ce
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.15.2
  Git commit:       4484c46d9d
  Built:            Sat Sep 26 12:03:35 2020
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          v1.4.1.m
  GitCommit:        c623d1b36f09f8ef6536a057bd658b3aa8632828.m
 runc:
  Version:          1.0.0-rc92
  GitCommit:        ff819c7e9184c13b7c2607fe6c30ae19403a7aff
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0
```

```
[root@Acheron archivebox]# docker-compose version
docker-compose version 1.27.4, build 40524192
docker-py version: 4.3.1
CPython version: 3.7.7
OpenSSL version: OpenSSL 1.1.0l  10 Sep 2019
```
diff --git a/archivebox/search/backends/sonic.py b/archivebox/search/backends/sonic.py
index f0beadd..f3ef662 100644
--- a/archivebox/search/backends/sonic.py
+++ b/archivebox/search/backends/sonic.py
@@ -5,7 +5,7 @@ from sonic import IngestClient, SearchClient
 from archivebox.util import enforce_types
 from archivebox.config import SEARCH_BACKEND_HOST_NAME, SEARCH_BACKEND_PORT, SEARCH_BACKEND_PASSWORD, SONIC_BUCKET, SONIC_COLLECTION
 
-MAX_SONIC_TEXT_LENGTH = 20000
+MAX_SONIC_TEXT_LENGTH = 2000
 
 @enforce_types
 def index(snapshot_id: str, texts: List[str]):
['archivebox/search/backends/sonic.py']