Add documentation for new YAML attributes

[skip ci]

docs/sections/cn_workflows.md

@@ -4,21 +4,11 @@ This section introduces the YAML syntax used by Popper, describes the
 workflow execution runtime and shows how to execute workflows in 
 alternative container engines.
 
-> _**NOTE**: Popper also supports the [now-deprecated HCL 
-> syntax][drophcl] that was introduced in the alpha version of [Github 
-> Action Workflows][parser]. We strongly recommend the use of Popper's 
-> own YAML syntax._
-
-[parser]: https://github.blog/2019-02-07-an-open-source-parser-for-github-actions/
-[drophcl]: https://github.blog/changelog/2019-09-17-github-actions-will-stop-running-workflows-written-in-hcl/
-
 ## Syntax
 
 A Popper workflow file looks like the following:
 
 ```yaml
-version: '1'
-
 steps:
 - uses: docker://alpine:3.9
   args: ["ls", "-la"]
@@ -44,15 +34,15 @@ options that are applied to the workflow.
 The following table describes the attributes that can be used for a 
 step. All attributes are optional with the exception of the `uses` attribute.
 
-| Attribute  | Description |
-| :--------- | :-------------------- |
-| `uses`     | The Docker image that will be executed for that step. For example,<br>`uses: docker://node:10`. See **"Referencing images in a step"** section below for more examples. |
-| `runs`     | Specifies the command to run in the docker image. If `runs` is omitted, the<br>command specified in the `Dockerfile`'s `ENTRYPOINT` instruction will execute.<br>Use the `runs` attribute when the `Dockerfile` does not specify an `ENTRYPOINT`<br>or you want to override the `ENTRYPOINT` command. The `runs` attribute does not<br>invoke a shell by default. Using `runs: "echo $VAR"` will not print the value<br>stored in `$VAR`, but will instead print `\"\$VAR.\"`. To use environment<br>variables with the `runs` instruction, you must include a shell to expand<br>the variables, for example: `runs: ["sh", "-c", "echo $VAR"]`.  If the value of `runs`<br>refers to a local script, the path is relative to the workspace folder (see [The workspace](#the-workspace) section below)|
-| `args`     | The arguments to pass to the command. This is an array of strings. For example,<br> `args: ["--flag", "--arg", "value"]`. If the value of `args`<br>refers to a local script, the path is relative to the workspace folder (see [The workspace](#the-workspace) section below). |
-| `env`      | The environment variables to set inside the container's runtime environment. If<br>you need to pass environment variables into a step, make sure it runs a command<br>shell to perform variable substitution. For example, if your `runs` attribute is<br>set to `["sh", "-c"]`, the value of `args` will be passed to `sh -c` and<br>executed in a command shell. Alternatively, if your `Dockerfile` uses an<br>`ENTRYPOINT` to run the same command (`"sh -c"`), `args` will execute in a<br>command shell as well. See [`ENTRYPOINT`](https://docs.docker.com/engine/reference/builder/#entrypoint) for more details. |
-| `secrets`  | Specifies the names of the secret variables to set in the runtime environment<br>which the container can access as an environment variable. For example,<br>`secrets: ["SECRET1", "SECRET2"]`. |
-| `id`       | Assigns an identifier to the step. By default, steps are asigned a numerid id<br>corresponding to the order of the step in the list, with `1` identifying<br>the first step. |
-| `needs`    | Identifies steps that must complete successfully before this step will be<br>invoked. It can be a string or an array of strings. |
+| Attribute   | Description |
+| :---------  | :-------------------- |
+| `uses`      | **required** The Docker image that will be executed for that step. For example,<br>`uses: docker://node:10`. See **"Referencing images in a step"** section below for<br>more examples. |
+| `id`        | **optional** Assigns an identifier to the step. By default, steps are asigned a numerid id<br>corresponding to the order of the step in the list, with `1` identifying<br>the first step. |
+| `runs`      | **optional** Specifies the command to run in the docker image. If `runs` is omitted, the<br>command specified in the `Dockerfile`'s `ENTRYPOINT` instruction will execute.<br>Use the `runs` attribute when the `Dockerfile` does not specify an `ENTRYPOINT`<br>or you want to override the `ENTRYPOINT` command. The `runs` attribute does not<br>invoke a shell by default. Using `runs: "echo $VAR"` will not print the value<br>stored in `$VAR`, but will instead print `\"\$VAR.\"`. To use environment<br>variables with the `runs` instruction, you must include a shell to expand<br>the variables, for example: `runs: ["sh", "-c", "echo $VAR"]`.  If the value of `runs`<br>refers to a local script, the path is relative to the workspace folder (see [The workspace](#the-workspace) section below)|
+| `args`      | **optional** The arguments to pass to the command. This is an array of strings. For example,<br> `args: ["--flag", "--arg", "value"]`. If the value of `args`<br>refers to a local script, the path is relative to the workspace folder (see [The workspace](#the-workspace) section below). Similarly to the `runs` attribute, if an envrionment<br>variable is being referenced, in order for this reference to be valid, a shell must be<br>be invoked (in the `runs` attribute) in order to expand the value of the variable. |
+| `env`       | **optional** The environment variables to set inside the container's runtime environment. If<br>you need to pass environment variables into a step, make sure it runs a command<br>shell to perform variable substitution. For example, if your `runs` attribute is<br>set to `["sh", "-c"]`, the value of `args` will be passed to `sh -c` and<br>executed in a command shell. Alternatively, if your `Dockerfile` uses an<br>`ENTRYPOINT` to run the same command (`"sh -c"`), `args` will execute in a<br>command shell as well. See [`ENTRYPOINT`](https://docs.docker.com/engine/reference/builder/#entrypoint) for more details. |
+| `secrets`   | **optional** Specifies the names of the secret variables to set in the runtime environment<br>which the container can access as an environment variable. For example,<br>`secrets: ["SECRET1", "SECRET2"]`. |
+| `skip_pull` | **optional** Assume that the given container image already exist and skip pulling it. |
 
 ### Referencing images in a step
 
@@ -157,7 +147,6 @@ For example, let's look at a workflow that writes to a `myfile` in the
 workspace:
 
 ```yaml
-version: '1'
 steps:
 - uses: docker://alpine:3.9
   args: [touch, ./myfile]
@@ -203,7 +192,6 @@ defines environment variables using the `env` attribute. For example,
 you could set the variables `FIRST`, `MIDDLE`, and `LAST` using this:
 
 ```yaml
-version: '1'
 steps:
 - uses: "docker://alpine:3.9"
   args: ["sh", "-c", "echo my name is: $FIRST $MIDDLE $LAST"]
@@ -287,7 +275,6 @@ value for the `uses` attribute. This value instructs Popper to execute
 the command or script given in the `runs` attribute. For example:
 
 ```yaml
-version: '1'
 steps:
 - uses: "sh"
   runs: ["ls", "-la"]
@@ -304,7 +291,6 @@ script specified in the `runs` attribute are NOT executed in a shell.
 If you need a shell, you have to explicitly invoke one, for example:
 
 ```yaml
-version: '1'
 steps:
 - uses: sh
   runs: [bash, -c, 'sleep 10 && true && exit 0']
@@ -339,7 +325,6 @@ several nodes. You can get started with running Popper workflows through Slurm b
 
 Let's consider a workflow `sample.yml` like the one shown below.
 ```yaml
-version: '1'
 steps:
 - id: one
   uses: docker://alpine:3.9