fix single ENV syntax

[NoumanSaleem]

Hi,

With our latest release using yesterday's container build, we've seen a spike in CPU usage for docker-cleanup.

CONTAINER           CPU %               MEM USAGE / LIMIT     MEM %               NET I/O             BLOCK I/O
9b5cb62b6772        109.55%             585.7 kB / 52.43 MB   1.12%               2.676 kB / 648 B    0 B / 3.092 MB

The latest change to move ENV to single line is missing required = signs as outlined in the docs, and causes the env vars to be incorrectly set: https://docs.docker.com/engine/reference/builder/#env

❯ docker run --rm -it --entrypoint=/bin/sh meltwater/docker-cleanup -c "printenv"
HOSTNAME=bd91acb68f4f
SHLVL=1
HOME=/root
TERM=xterm
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
CLEAN_PERIOD=**None** DELAY_TIME **None** KEEP_IMAGES **None** KEEP_CONTAINERS **None** LOOP true DEBUG 0
PWD=/

This PR shows env correctly:

❯ docker run --rm -it --entrypoint=/bin/sh cleanup -c "printenv"
DEBUG=0
HOSTNAME=a7649a49780d
SHLVL=1
HOME=/root
KEEP_CONTAINERS=**None**
LOOP=true
TERM=xterm
DELAY_TIME=**None**
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
CLEAN_PERIOD=**None**
PWD=/
KEEP_IMAGES=**None**

thanks