Script mode set -xe by default when no shebang used

Fixes #1735

When I use script mode without a #! the current behaviour is to insert
one for me - `#!/bin/sh`. This is helpful and removes an unneccessary
line from my script. In addition I'd like two things by default: First
I want the script to error out if any of the commands error out. Second
I'd like to see the commands the script runs before their output. These
two features are baked into `sh` with the `set -xe` flags.

After this change the default behaviour of script mode when no shebang
is provided is to insert both the default shebang as well as the flags
required to make shell errors and shell echos happen.

docs/tasks.md

@@ -156,11 +156,16 @@ If this field is present, the step cannot specify `command`.
 When specified, a `script` gets invoked as if it were the contents of a file in
 the container. Any `args` are passed to the script file.
 
-Scripts that do not start with a shebang
-[shebang](https://en.wikipedia.org/wiki/Shebang_(Unix)) line will use a default
-value of `#!/bin/sh`, although users can override this by starting their script
-with a shebang to declare what tool should be used to interpret the script.
-That tool must then also be available within the step's container.
+Scripts that do not start with a [shebang](https://en.wikipedia.org/wiki/Shebang_(Unix))
+line will use the following default preamble:
+
+```bash
+#!/bin/sh
+set -xe
+```
+Users can override this by starting their script with a shebang to declare what
+tool should be used to interpret the script. That tool must then also be
+available within the step's container.
 
 This allows you to execute a Bash script, if the image includes `bash`:
 

pkg/pod/script.go

@@ -27,9 +27,9 @@ import (
 )
 
 const (
-	scriptsVolumeName = "scripts"
-	scriptsDir        = "/tekton/scripts"
-	defaultShebang    = "#!/bin/sh\n"
+	scriptsVolumeName     = "scripts"
+	scriptsDir            = "/tekton/scripts"
+	defaultScriptPreamble = "#!/bin/sh\nset -xe\n"
 )
 
 var (
@@ -77,7 +77,7 @@ func convertScripts(shellImage string, steps []v1alpha1.Step) (*corev1.Container
 
 		script := s.Script
 		if !hasShebang {
-			script = defaultShebang + s.Script
+			script = defaultScriptPreamble + s.Script
 		}
 
 		// At least one step uses a script, so we should return a

pkg/pod/script_test.go

@@ -101,6 +101,7 @@ tmpfile="/tekton/scripts/script-3-j2tds"
 touch ${tmpfile} && chmod +x ${tmpfile}
 cat > ${tmpfile} << 'script-heredoc-randomly-generated-vr6ds'
 #!/bin/sh
+set -xe
 no-shebang
 script-heredoc-randomly-generated-vr6ds
 `},