Docker container eating space over time in my EC2

I've installed only Hasura and a Postgres database (yes yes, I'll change it) in 1 docker container each. The containers themselves don't use much space, but my overall storage space is rapidly being used up each passing day! I have no idea why, I'm pretty new to this.

Using Ubuntu 16.0.4 in AWS EC2.

Results of docker ps as

015ff0c04b91        hasura/graphql-engine:v2.2.1   "graphql-engine serve"   11 days ago         Up 3 minutes        0.0.0.0:8080->8080/tcp   ubuntu_graphql-engine_1         
 0B (virtual 395MB)
b79e1cfd6c3e        postgres:12                    "docker-entrypoint.s…"   7 weeks ago         Up 11 days          0.0.0.0:5432->5432/tcp   b79e1cfd6c3e_ubuntu_postgres_1  
 135B (virtual 373MB)

Results of : docker system df

TYPE                TOTAL               ACTIVE              SIZE                RECLAIMABLE
Images              2                   2                   767.5MB             0B (0%)
Containers          2                   2                   135B                0B (0%)
Local Volumes       1                   1                   193.8MB             0B (0%)
Build Cache         0                   0                   0B                  0B

Results of df -h

Filesystem      Size  Used Avail Use% Mounted on
udev            992M     0  992M   0% /dev
tmpfs           200M   21M  179M  11% /run
/dev/xvda1       20G   15G  4.6G  77% /
tmpfs          1000M     0 1000M   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs          1000M     0 1000M   0% /sys/fs/cgroup
/dev/loop0       44M   44M     0 100% /snap/snapd/15177
/dev/loop1       44M   44M     0 100% /snap/snapd/14978
/dev/loop2       56M   56M     0 100% /snap/core18/2284
/dev/loop3       25M   25M     0 100% /snap/amazon-ssm-agent/4046
/dev/loop4       27M   27M     0 100% /snap/amazon-ssm-agent/5163
/dev/loop5       56M   56M     0 100% /snap/core18/2344
tmpfs           200M     0  200M   0% /run/user/1000

Results of : sudo du -x -h / | sort -h | tail -10

343M/usr/bin
444M/usr/lib
783M/var/lib/docker/overlay2
1.2G/usr
12G/var/lib/docker/containers
12G/var/lib/docker/containers/015ff0c04b91075472bceeb30f0fac1ef0e7b50518241047138a6d184b2897df
13G/var/lib/docker
14G/var
14G/var/lib
15G/

Results of docker info

Containers: 2
 Running: 2
 Paused: 0
 Stopped: 0
Images: 2
Server Version: 18.09.7
Storage Driver: overlay2
 Backing Filesystem: extfs
 Supports d_type: true
 Native Overlay Diff: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: bridge host macvlan null overlay
 Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog

I've already attempted:

• docker system prune (0B reclaimed)
• docker restart
• docker-compose up detach build

No effect on reducing storage space.