How to debug "no route to host" in a docker container - docker

Question

I just don't have enough networking knowledge to understand this.
On my laptop, I'm running both Docker and multiple vagrant VMs.
I want to connect to one of the vagrant VMs from within a docker container but ping keeps hanging or spitting out "Destination Host Unreachable". I can ping the vagrant VMs just fine from the host (ie. outside the container).
Could you point me in the right direction to fixing this? I basically want to install nginx on the vagrant VMs but have some load balancers in Docker.
This means that docker containers need to be able to "see" the vagrant VMs.
Do I need a route table entry? Do I need a special network adapter? Do I need to create a bridge? I just don't know enough and would appreciate a nudge in the right direction.
Here are details from the container:
root#d755dbb8bbc9:/# ip route
default via 172.18.0.1 dev eth1
10.0.1.0/24 dev eth2 proto kernel scope link src 10.0.1.6
10.255.0.0/16 dev eth0 proto kernel scope link src 10.255.0.4
172.18.0.0/16 dev eth1 proto kernel scope link src 172.18.0.5
root#d755dbb8bbc9:/# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet 10.255.0.30/32 brd 10.255.0.30 scope global lo
valid_lft forever preferred_lft forever
inet 10.0.1.41/32 brd 10.0.1.41 scope global lo
valid_lft forever preferred_lft forever
2: tunl0#NONE: <NOARP> mtu 1480 qdisc noop state DOWN group default qlen 1
link/ipip 0.0.0.0 brd 0.0.0.0
3: ip6tnl0#NONE: <NOARP> mtu 1452 qdisc noop state DOWN group default qlen 1
link/tunnel6 :: brd ::
29: eth0#if30: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc noqueue state UP group default
link/ether 02:42:0a:ff:00:04 brd ff:ff:ff:ff:ff:ff link-netnsid 0
inet 10.255.0.4/16 brd 10.255.255.255 scope global eth0
valid_lft forever preferred_lft forever
35: eth1#if36: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default
link/ether 02:42:ac:12:00:05 brd ff:ff:ff:ff:ff:ff link-netnsid 1
inet 172.18.0.5/16 brd 172.18.255.255 scope global eth1
valid_lft forever preferred_lft forever
39: eth2#if40: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc noqueue state UP group default
link/ether 02:42:0a:00:01:06 brd ff:ff:ff:ff:ff:ff link-netnsid 2
inet 10.0.1.6/24 brd 10.0.1.255 scope global eth2
valid_lft forever preferred_lft forever
And here is some stuff from on of the vagrant VMs:
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
link/ether 08:00:27:cf:1a:c3 brd ff:ff:ff:ff:ff:ff
inet 10.0.2.15/24 brd 10.0.2.255 scope global dynamic eth0
valid_lft 67730sec preferred_lft 67730sec
inet6 fe80::a00:27ff:fecf:1ac3/64 scope link
valid_lft forever preferred_lft forever
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
link/ether 08:00:27:ca:c7:a1 brd ff:ff:ff:ff:ff:ff
inet 172.17.8.101/16 brd 172.17.255.255 scope global eth1
valid_lft forever preferred_lft forever
inet6 fe80::a00:27ff:feca:c7a1/64 scope link
valid_lft forever preferred_lft forever
core#core-01 ~ $ ip route
default via 10.0.2.2 dev eth0 proto dhcp src 10.0.2.15 metric 1024
10.0.2.0/24 dev eth0 proto kernel scope link src 10.0.2.15
10.0.2.2 dev eth0 proto dhcp scope link src 10.0.2.15 metric 1024
172.17.0.0/16 dev eth1 proto kernel scope link src 172.17.8.101
When I ping 172.17.8.101 (the ip of the vagrant VM i want to ping) from the docker container, it just hangs. How can I get access to one of the VMs from one of the docker containers?