### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

When running localstack not in docker, but in an environment where `/proc/1/cgroup` exists, `in_docker()` at https://github.com/localstack/localstack/blob/d806081ab4994b8805251388e1f66140b96eb46a/localstack/config.py#L565 attempts to open `/etc/hostname` without any checks for its existence. In an environment where it does not exist, the code exits with an error instead of proceeding.

### Expected Behavior

`in_docker()` should either proceed without checking for hostname in `content`, or get the hostname from a safer canonical source, such as:

```python
import socket
hostname = socket.gethostname()
```


### How are you starting LocalStack?

With the `localstack` script

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

```ruby
  LOCAL_STACK = ProcessHelper::ProcessHelper.new(print_lines: true)
  LOCAL_STACK.start(
    ['pipenv', 'run', 'env', 'SERVICES=sns,sqs,s3', 'localstack', 'start', '--host'],
    /Ready/,
    60
  )
```

#### Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

Failure occurs in `config.py` before any client commands get a chance to run.


### Environment

```markdown
- OS: CentOS Linux release 7.9.2009 (Core)
- LocalStack: 0.13.0.9
```


### Anything else?

Output on failure, from Jenkins:
```
11:22:13      __                     _______ __             __
11:22:13     / /   ____  _________ _/ / ___// /_____ ______/ /__
11:22:13    / /   / __ \/ ___/ __ `/ /\__ \/ __/ __ `/ ___/ //_/
11:22:13   / /___/ /_/ / /__/ /_/ / /___/ / /_/ /_/ / /__/ ,<
11:22:13  /_____/\____/\___/\__,_/_//____/\__/\__,_/\___/_/|_|
11:22:13 
11:22:13  💻 LocalStack CLI 0.13.0.9
11:22:13 
11:22:16 Traceback (most recent call last):
11:22:16   File "/home/jenkins/.local/share/virtualenvs/dsl.python_cosmos_lambda.distiller.build-uIfsvcZ_/bin/localstack", line 23, in <module>
11:22:16     main()
11:22:16   File "/home/jenkins/.local/share/virtualenvs/dsl.python_cosmos_lambda.distiller.build-uIfsvcZ_/bin/localstack", line 19, in main
11:22:16     main.main()
11:22:16   File "/home/jenkins/.local/share/virtualenvs/dsl.python_cosmos_lambda.distiller.build-uIfsvcZ_/lib/python3.7/site-packages/localstack/cli/main.py", line 6, in main
11:22:16     cli()
11:22:16   File "/home/jenkins/.local/share/virtualenvs/dsl.python_cosmos_lambda.distiller.build-uIfsvcZ_/lib/python3.7/site-packages/localstack/cli/plugin.py", line 15, in __call__
11:22:16     self.group(*args, **kwargs)
11:22:16   File "/home/jenkins/.local/share/virtualenvs/dsl.python_cosmos_lambda.distiller.build-uIfsvcZ_/lib/python3.7/site-packages/click/core.py", line 1128, in __call__
11:22:16     return self.main(*args, **kwargs)
11:22:16   File "/home/jenkins/.local/share/virtualenvs/dsl.python_cosmos_lambda.distiller.build-uIfsvcZ_/lib/python3.7/site-packages/click/core.py", line 1053, in main
11:22:16     rv = self.invoke(ctx)
11:22:16   File "/home/jenkins/.local/share/virtualenvs/dsl.python_cosmos_lambda.distiller.build-uIfsvcZ_/lib/python3.7/site-packages/click/core.py", line 1659, in invoke
11:22:16     return _process_result(sub_ctx.command.invoke(sub_ctx))
11:22:16   File "/home/jenkins/.local/share/virtualenvs/dsl.python_cosmos_lambda.distiller.build-uIfsvcZ_/lib/python3.7/site-packages/click/core.py", line 1395, in invoke
11:22:16     return ctx.invoke(self.callback, **ctx.params)
11:22:16   File "/home/jenkins/.local/share/virtualenvs/dsl.python_cosmos_lambda.distiller.build-uIfsvcZ_/lib/python3.7/site-packages/click/core.py", line 754, in invoke
11:22:16     return __callback(*args, **kwargs)
11:22:16   File "/home/jenkins/.local/share/virtualenvs/dsl.python_cosmos_lambda.distiller.build-uIfsvcZ_/lib/python3.7/site-packages/localstack/cli/localstack.py", line 105, in cmd_start
11:22:16     from localstack.utils import bootstrap
11:22:16   File "/home/jenkins/.local/share/virtualenvs/dsl.python_cosmos_lambda.distiller.build-uIfsvcZ_/lib/python3.7/site-packages/localstack/utils/bootstrap.py", line 11, in <module>
11:22:16     from localstack import config, constants
11:22:16   File "/home/jenkins/.local/share/virtualenvs/dsl.python_cosmos_lambda.distiller.build-uIfsvcZ_/lib/python3.7/site-packages/localstack/config.py", line 572, in <module>
11:22:16     is_in_docker = in_docker()
11:22:16   File "/home/jenkins/.local/share/virtualenvs/dsl.python_cosmos_lambda.distiller.build-uIfsvcZ_/lib/python3.7/site-packages/localstack/config.py", line 565, in in_docker
11:22:16     os_hostname = open("/etc/hostname", "rt").read().strip()
11:22:16 FileNotFoundError: [Errno 2] No such file or directory: '/etc/hostname'
11:22:16 EOF encountered while waiting for output that matches '(?-mix:Ready)' (RuntimeError)
```
import logging
import os
import platform
import re
import socket
import subprocess
import tempfile
import time
from typing import Any, Dict, List, Mapping, Tuple
import six
from boto3 import Session
from localstack.constants import (
    AWS_REGION_US_EAST_1,
    DEFAULT_BUCKET_MARKER_LOCAL,
    DEFAULT_DEVELOP_PORT,
    DEFAULT_LAMBDA_CONTAINER_REGISTRY,
    DEFAULT_PORT_EDGE,
    DEFAULT_SERVICE_PORTS,
    FALSE_STRINGS,
    INSTALL_DIR_INFRA,
    LOCALHOST,
    LOCALHOST_IP,
    LOG_LEVELS,
    TRACE_LOG_LEVELS,