diff --git a/cibuildwheel/environment.py b/cibuildwheel/environment.py
index d0e14a02..9f7ea2c4 100644
--- a/cibuildwheel/environment.py
+++ b/cibuildwheel/environment.py
@@ -1,6 +1,6 @@
 import bashlex  # type: ignore
 
-from typing import Dict, List, MutableMapping
+from typing import Dict, List, Mapping
 
 from . import bashlex_eval
 
@@ -61,7 +61,7 @@ class ParsedEnvironment:
     def __init__(self, assignments: List[EnvironmentAssignment]):
         self.assignments = assignments
 
-    def as_dictionary(self, prev_environment: MutableMapping[str, str]) -> Dict[str, str]:
+    def as_dictionary(self, prev_environment: Mapping[str, str]) -> Dict[str, str]:
         environment = dict(**prev_environment)
 
         for assignment in self.assignments:
diff --git a/cibuildwheel/linux.py b/cibuildwheel/linux.py
index aa672f7b..aa620f21 100644
--- a/cibuildwheel/linux.py
+++ b/cibuildwheel/linux.py
@@ -134,22 +134,24 @@ def build(options: BuildOptions) -> None:
             call(['docker', 'start', container_name])
 
             if options.before_all:
-                before_all_prepared = prepare_command(options.before_all, project='/project', package=options.package_dir)
-                task = """
-                PS4='    + '
+                call(
+                    ['docker', 'exec', '-i', container_name] + shell_cmd, 
+                    universal_newlines=True,
+                    input='''
+                        PS4='    + '
 
-                set -o errexit
-                set -o xtrace
+                        set -o errexit
+                        set -o xtrace
 
-                {environment_exports}
+                        {environment_exports}
 
-                {before_all_prepared}
+                        sh -c {before_all}
 
-                """.format(
-                    environment_exports='\n'.join(options.environment.as_shell_commands()),
-                    before_all_prepared=before_all_prepared
+                        '''.format(
+                            environment_exports='\n'.join(options.environment.as_shell_commands()),
+                            before_all=prepare_command(options.before_all, project='/project', package=container_package_dir)
+                        )
                 )
-                call(['docker', 'exec', '-i', container_name] + shell_cmd, universal_newlines=True, input=task)
 
             for config in platform_configs:
                 if options.dependency_constraints: