Docker-Compose Build Failure

This may be related to Issue #247 but I wanted to post based on how I am setup.

I'm using Docker-Compose to attempt to build caddy and it keeps failing. I have the same configuration running on a different system, but I haven't rebuilt in a while so it worked back when I originally got it setup.

```
version: '3.4'
services:
caddy:
build:
context: github.com/abiosoft/caddy-docker.git
args:
plugins: cgi,cors,webdav
container_name: "caddy"
environment:
- ACME_AGREE=true
ports:
- "80:80"
- "443:443"
volumes:
- /mnt/data/disk2/caddy/Caddyfile:/etc/Caddyfile:ro
- /mnt/data/disk2/caddy/www:/var/www:rw
- /mnt/data/disk2/caddy/cert:/root/.caddy:rw
- /mnt/data/disk2/caddy/logs:/root/logs:rw
restart: always
```

When I run my `docker-compose up -d` I get through most of the stages until it goes to build caddy.

```
starting stage: building caddy
go: downloading gopkg.in/natefinch/lumberjack.v2 v2.0.0
go: downloading github.com/hashicorp/go-syslog v1.0.0
go: downloading github.com/jimstudt/http-authentication v0.0.0-20140401203705-3eca13d6893a
go: downloading gopkg.in/yaml.v2 v2.2.2
go: downloading github.com/gorilla/websocket v1.4.0
go: downloading github.com/dustin/go-humanize v1.0.0
go: downloading github.com/russross/blackfriday v0.0.0-20170610170232-067529f716f4
go: downloading github.com/naoina/toml v0.1.1
go: downloading github.com/lucas-clemente/quic-go v0.10.2
go: extracting github.com/jimstudt/http-authentication v0.0.0-20140401203705-3eca13d6893a
go: extracting github.com/hashicorp/go-syslog v1.0.0
go: extracting github.com/russross/blackfriday v0.0.0-20170610170232-067529f716f4
go: extracting gopkg.in/yaml.v2 v2.2.2
go: extracting github.com/naoina/toml v0.1.1
go: extracting gopkg.in/natefinch/lumberjack.v2 v2.0.0
go: extracting github.com/gorilla/websocket v1.4.0
go: downloading github.com/naoina/go-stringutil v0.1.0
go: extracting github.com/dustin/go-humanize v1.0.0
go: extracting github.com/lucas-clemente/quic-go v0.10.2
go: extracting github.com/naoina/go-stringutil v0.1.0
go: downloading github.com/bifurcation/mint v0.0.0-20180715133206-93c51c6ce115
go: downloading github.com/cheekybits/genny v0.0.0-20170328200008-9127e812e1e9
go: downloading github.com/lucas-clemente/aes12 v0.0.0-20171027163421-cd47fb39b79f
go: downloading github.com/hashicorp/golang-lru v0.0.0-20180201235237-0fb14efe8c47
go: downloading github.com/lucas-clemente/quic-go-certificates v0.0.0-20160823095156-d2f86524cced
go: extracting github.com/cheekybits/genny v0.0.0-20170328200008-9127e812e1e9
go: extracting github.com/hashicorp/golang-lru v0.0.0-20180201235237-0fb14efe8c47
go: extracting github.com/lucas-clemente/quic-go-certificates v0.0.0-20160823095156-d2f86524cced
go: extracting github.com/lucas-clemente/aes12 v0.0.0-20171027163421-cd47fb39b79f
go: extracting github.com/bifurcation/mint v0.0.0-20180715133206-93c51c6ce115
can't load package: package caddy:
cgi.go:2:10: invalid import path: ""
error at 'building caddy'
ERROR: Service 'caddy' failed to build: The command '/bin/sh -c VERSION=${version} PLUGINS=${plugins} ENABLE_TELEMETRY=${enable_telemetry} /bin/sh /usr/bin/builder.sh' returned a non-zero code: 1

```