Add shell script linting with shellcheck for better portability

docs/content/contributing/building-testing.md

@@ -43,7 +43,7 @@ $ ls dist/
 traefik*
 ```
 
-The following targets can be executed outside Docker (we don't recommend that):
+The following targets can be executed outside Docker by setting the variable `PRE_TARGET` to an empty string (we don't recommend that):
 
 - `test-unit`
 - `test-integration`
@@ -61,24 +61,24 @@ PRE_TARGET= make test-unit
 You need `go` v1.12+.
 
 !!! tip "Source Directory"
- 
+
     It is recommended that you clone Traefik into the `~/go/src/github.com/containous/traefik` directory.
     This is the official golang workspace hierarchy that will allow dependencies to be properly resolved.
 
 !!! note "Environment"
 
     Set your `GOPATH` and `PATH` variable to be set to `~/go` via:
-    
+
     ```bash
     export GOPATH=~/go
     export PATH=$PATH:$GOPATH/bin
     ```
- 
+
     For convenience, add `GOPATH` and `PATH` to your `.bashrc` or `.bash_profile`
-    
+
     Verify your environment is setup properly by running `$ go env`.
     Depending on your OS and environment, you should see an output similar to:
-    
+
     ```bash
     GOARCH="amd64"
     GOBIN=""