Fix Dockerfile compatibility with Alpine Linux

[jordanjennings]

In order to support Alpine Linux the CGO_ENABLED=0 flag is needed, which is set in .goreleaser.yml but was not set in the Dockerfile.

Before this change the binary built via the Dockerfile was not compatible with the hashicorp/terraform base image, which is based on Alpine Linux.

In other words this is what happened if you tried to build master and use it directly via the Dockerfile:

❯ git checkout master
Switched to branch 'master'
Your branch is up to date with 'origin/master'.

❯ docker build -t jordanjennings/databricks-terraform:master .
Sending build context to Docker daemon  20.83MB
Step 1/9 : FROM golang:1.13

... lots of build output removed ...

Successfully built ed0089b8fd5d
Successfully tagged jordanjennings/databricks-terraform:master

❯ docker run --rm -it --entrypoint sh jordanjennings/databricks-terraform:master

/ # ls -al ~/.terraform.d/plugins/
total 30044
drwxr-xr-x    2 root     root          4096 Jun  2 20:33 .
drwxr-xr-x    3 root     root          4096 Jun  2 20:33 ..
-rwxr-xr-x    1 root     root      30755834 Jun  2 20:33 terraform-provider-databricks

/ # ~/.terraform.d/plugins/terraform-provider-databricks 
sh: /root/.terraform.d/plugins/terraform-provider-databricks: not found

Here's the same thing with the CGO_ENABLED=0 flag as included in this PR (error message as expected):

❯ docker run --rm -it --entrypoint sh jordanjennings/databricks-terraform:pr
/ # ~/.terraform.d/plugins/terraform-provider-databricks 
2020/06/03 14:28:39 dev, commit , built at unknown
This binary is a plugin. These are not meant to be executed directly.
Please execute the program that consumes these plugins, which will
load any plugins automatically

I don't know if CGO_ENABLED=0 is something that should instead be set in the Makefile, but setting in the Dockerfile was the more isolated way to handle it here.

[codecov-commenter]

Codecov Report

Merging #80 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #80   +/-   ##
=======================================
  Coverage   48.74%   48.74%           
=======================================
  Files          44       44           
  Lines        5943     5943           
=======================================
  Hits         2897     2897           
  Misses       2993     2993           
  Partials       53       53           

Flag	Coverage Δ
#client	88.78% <ø> (ø)
#provider	37.22% <ø> (ø)

[stikkireddy]

Hey @jordanjennings good find, I was able to recreate the issue: it seems to be related to one of the packages: "net". You can find more info here: https://stackoverflow.com/questions/36279253/go-compiled-binary-wont-run-in-an-alpine-docker-container-on-ubuntu-host

It seems turning of CGO in the build process is one way of fixing this. The client library heavily uses the net library for api calls.

Thanks again for the contribution, LGTM so merging! 👍