Problem when configuring cache_dir for squid proxy

squid-cache_dir.txt

1. requirement: setup squid proxy server using docker.

2. The problem:  cannot configure cache_dir for squid.

3. More detail: 
- I tried some docker image to setup squid:
https://hub.docker.com/r/babim/squid/ (babin/squid:auth)
https://hub.docker.com/r/scbunn/squid/ (scbunn/squid:3.5.17)
all of them work fine when I DON'T CONFIGURE CACHE_DIR for squid.
but when I configure CACHE_DIR, the squid process was killed immediately after starting.

Then I decided to create my own image to be easy develop and maintain as  I understand all about it.
I created a docker image here: https://hub.docker.com/r/swdream/squid/
Github repo: https://github.com/swdream/squid

but I got the same problem:
when I configure CACHE_DIR, the squid process was killed immediately after starting
https://github.com/swdream/squid/blob/master/conf/squid.conf#L27

with no error log is printed out:
```
thanhnguyen@thanhnguyen:~/github/squid$ docker run --name squid -p 3128:3128 -e USERNAME=foo -e PASSWORD=bar -v /data/cache:/var/cache/squid swdream/squid:3.5.23
thanhnguyen@thanhnguyen:~/github/squid$ docker ps -a | grep squid
996dfbc71f94        swdream/squid:3.5.23             "/entrypoint.sh squid"   8 seconds ago       Exited (1) 7 seconds ago                                       squid
...
thanhnguyen@thanhnguyen:~/github/squid$ docker logs 996dfbc71f94
Adding password for user foo
Staring squid [3.5.23]
```

4. How to reproduce:
In your machine, run this docker command to use my image swdream/squid:3.5.23
```
$ docker run --name squid -p 3128:3128 -e USERNAME=foo -e PASSWORD=bar -v /data/cache:/var/cache/squid swdream/squid:3.5.23
```
then, cannot start squid container and squid process will be killed immediately after starting with no error log found.

5. What I did:
-  add some checks to make sure all necessary directories are already existed before stating squid and they have right owner.
https://github.com/swdream/squid/blob/68835126c3a82370d6edf85036f1482f984b5553/entrypoint.sh#L15
https://github.com/swdream/squid/blob/68835126c3a82370d6edf85036f1482f984b5553/entrypoint.sh#L24
https://github.com/swdream/squid/blob/68835126c3a82370d6edf85036f1482f984b5553/entrypoint.sh#L38

- make sure start squid process with right user:
https://github.com/swdream/squid/commit/1bedce2cab448a75969f67f29adebbd34cf3f88c

- Run manually all my commands in Dockerfile and entrypoint in a alpine 3.3 container (my image is based on alpine:3.3)
and squid worked find with the same config file.
https://github.com/swdream/squid/blob/master/conf/squid.conf#L27
```
/ # ps -ef | grep squid
  112 root       0:00 /usr/sbin/squid
  114 squid      0:01 {squid} (squid-1)
  115 squid      0:00 {log_file_daemon} (logfile-daemon) /var/log/squid/access.log
  121 root       0:00 grep squid
/ # grep -rn cache_dir /etc/squid/squid.conf
27:cache_dir aufs /var/cache/squid 100 16 256
```