Can't build container

`docker build` is failing for me today with the error: `go: github.com/mholt/caddy@v1.0.1: parsing go.mod: unexpected module path "github.com/caddyserver/caddy"
`

Full logs below:

```
$ docker build -t thepatrick/webserver-caddy --build-arg plugins=cloudflare,route53 github.com/abiosoft/caddy-docker.git 1
Sending build context to Docker daemon 109.1kB
Step 1/24 : FROM abiosoft/caddy:builder as builder
---> 5e3f22e6d0be
Step 2/24 : ARG version="1.0.1"
---> Running in 2f67cc2ebd63
Removing intermediate container 2f67cc2ebd63
---> 7244e5b4163a
Step 3/24 : ARG plugins="git,cors,realip,expires,cache,cloudflare"
---> Running in 5b144e79dff0
Removing intermediate container 5b144e79dff0
---> dde9d244d51f
Step 4/24 : ARG enable_telemetry="true"
---> Running in 2c76d5ef505c
Removing intermediate container 2c76d5ef505c
---> 6f2d65df036a
Step 5/24 : RUN go get -v github.com/abiosoft/parent
---> Running in cb639dfd59b1
github.com/abiosoft/parent (download)
github.com/abiosoft/parent
Removing intermediate container cb639dfd59b1
---> 5e34135da33e
Step 6/24 : RUN VERSION=${version} PLUGINS=${plugins} ENABLE_TELEMETRY=${enable_telemetry} /bin/sh /usr/bin/builder.sh
---> Running in 6228775928e1

starting stage: fetching caddy source
Cloning into '/go/src/github.com/mholt/caddy'...
Note: checking out 'f6ee100bae9c0de566a2f7ae531113be0a9a0b83'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

git checkout -b

Cloning into '/dnsproviders'...
finished stage: fetching caddy source ✓

starting stage: installing plugin helper
go get: warning: modules disabled by GO111MODULE=auto in GOPATH/src;
ignoring go.mod;
see 'go help modules'
github.com/abiosoft/caddyplug (download)
github.com/abiosoft/errs (download)
github.com/fatih/color (download)
Fetching https://golang.org/x/sys/unix?go-get=1
Parsing meta tags from https://golang.org/x/sys/unix?go-get=1 (status code 200)
get "golang.org/x/sys/unix": found meta tag get.metaImport{Prefix:"golang.org/x/sys", VCS:"git", RepoRoot:"https://go.googlesource.com/sys"} at https://golang.org/x/sys/unix?go-get=1
get "golang.org/x/sys/unix": verifying non-authoritative meta tag
Fetching https://golang.org/x/sys?go-get=1
Parsing meta tags from https://golang.org/x/sys?go-get=1 (status code 200)
golang.org/x/sys (download)
github.com/abiosoft/errs
github.com/fatih/color/vendor/github.com/mattn/go-isatty
golang.org/x/sys/unix
github.com/abiosoft/caddyplug/internal
github.com/fatih/color/vendor/github.com/mattn/go-colorable
github.com/fatih/color
github.com/abiosoft/caddyplug/caddyplug
finished stage: installing plugin helper ✓

starting stage: generating plugins
finished stage: generating plugins ✓

starting stage: customising plugins and telemetry
go: creating new go.mod: module caddy
go: finding github.com/mholt/caddy v1.0.1
go: github.com/mholt/caddy@v1.0.1: parsing go.mod: unexpected module path "github.com/caddyserver/caddy"
go: error loading module requirements
finished stage: customising plugins and telemetry ✓

starting stage: building caddy
go: finding github.com/go-acme/lego/providers/dns/route53 latest
go: finding github.com/go-acme/lego/providers/dns/cloudflare latest
go: finding github.com/mholt/caddy/caddy/caddymain latest
go: finding github.com/caddyserver/caddy/caddytls latest
go: finding github.com/mholt/caddy/caddy latest
go: downloading github.com/mholt/caddy v1.0.1
go: extracting github.com/mholt/caddy v1.0.1
go: finding github.com/caddyserver/caddy v1.0.1
go: finding github.com/go-acme/lego/providers/dns latest
go: finding github.com/go-acme/lego/providers latest
go: downloading github.com/caddyserver/caddy v1.0.1
go: extracting github.com/caddyserver/caddy v1.0.1
go: finding github.com/go-acme/lego v2.6.0+incompatible
go: downloading github.com/go-acme/lego v2.6.0+incompatible
go: extracting github.com/go-acme/lego v2.6.0+incompatible
go: github.com/mholt/caddy@v1.0.1: parsing go.mod: unexpected module path "github.com/caddyserver/caddy"
go: error loading module requirements
error at 'building caddy'
```