fix: set sh as default shell for containers (#853)

* fix: set default shell for containers to sh

This matches GitHub Actions behaviour where,
runners use bash since it's guaranteed to exist there
while containers use sh for compatibility

* tests: fixup for default shell

act/runner/testdata/container-hostname/push.yml

@@ -1,6 +1,10 @@
 name: container-hostname
 on: push
 
+defaults:
+  run:
+    shell: bash
+
 jobs:
   with-hostname:
     runs-on: ubuntu-latest

act/runner/testdata/shells/bash/push.yml

@@ -5,6 +5,19 @@ jobs:
     steps:
       - shell: bash
         run: |
-          if [[ -n "$PATH" ]] ; then
-            echo "I'm Bash!"
+          if [[ -n "$BASH" ]]; then
+            echo "I'm $BASH!"
+          else
+            exit 1
+          fi
+  check-container:
+    runs-on: ubuntu-latest
+    container: node:12-buster-slim
+    steps:
+      - shell: bash
+        run: |
+          if [[ -n "$BASH" ]]; then
+            echo "I'm $BASH!"
+          else
+            exit 1
           fi

act/runner/testdata/shells/defaults/push.yml

@@ -0,0 +1,21 @@
+on: push
+jobs:
+  check: # GHA uses `bash` as default for runners
+    runs-on: ubuntu-latest
+    steps:
+      - run: |
+          if [[ -n "$BASH" ]]; then
+            echo "I'm $BASH!"
+          else
+            exit 1
+          fi
+  check-container: # GHA uses `sh` as default for containers
+    runs-on: ubuntu-latest
+    container: alpine:latest
+    steps:
+      - run: |
+          if [ -z ${BASH+x} ]; then
+            echo "I'm sh!"
+          else
+            exit 1
+          fi

act/runner/testdata/shells/pwsh/push.yml

@@ -6,3 +6,10 @@ jobs:
       - shell: pwsh
         run: |
           $PSVersionTable
+  check-container:
+    runs-on: ubuntu-latest
+    container: ghcr.io/justingrote/act-pwsh:latest
+    steps:
+      - shell: pwsh
+        run: |
+          $PSVersionTable

act/runner/testdata/shells/python/push.yml

@@ -7,3 +7,11 @@ jobs:
         run: |
           import platform
           print(platform.python_version())
+  check-container:
+    runs-on: ubuntu-latest
+    container: node:12-buster
+    steps:
+      - shell: python
+        run: |
+          import platform
+          print(platform.python_version())

act/runner/testdata/shells/sh/push.yml

@@ -5,6 +5,19 @@ jobs:
     steps:
       - shell: sh
         run: |
-          if [ -n "$PATH" ] ; then
+          if [ -z ${BASH+x} ]; then
             echo "I'm sh!"
+          else
+            exit 1
+          fi
+  check-container:
+    runs-on: ubuntu-latest
+    container: alpine:latest
+    steps:
+      - shell: sh
+        run: |
+          if [ -z ${BASH+x} ]; then
+            echo "I'm sh!"
+          else
+            exit 1
           fi