Forward proxy ENV variables to proxy container

Multiple users have reported `dependabot-action` not respecting their
proxy settings. We had not been passing those along to our proxy.

__tests__/proxy-integration.test.ts

@@ -97,4 +97,30 @@ integration('ProxyBuilder', () => {
     const stdout = proc.stdout.toString()
     expect(stdout).toEqual('ca-pem-contents')
   })
+
+  jest.setTimeout(20000)
+  it('forwards custom proxy urls if configured', async () => {
+    process.env.HTTP_PROXY = 'HTTP_PROXY'
+
+    const proxy = await builder.run(jobId, credentials)
+    await proxy.container.start()
+
+    const id = proxy.container.id
+    const proc = spawnSync('docker', ['exec', id, 'printenv', 'HTTP_PROXY'])
+    const output = proc.stdout.toString()
+    expect(output).toEqual('HTTP_PROXY')
+  })
+
+  jest.setTimeout(20000)
+  it('forwards downcased proxy urls if configured', async () => {
+    process.env.https_proxy = 'https_proxy'
+
+    const proxy = await builder.run(jobId, credentials)
+    await proxy.container.start()
+
+    const id = proxy.container.id
+    const proc = spawnSync('docker', ['exec', id, 'printenv', 'https_proxy'])
+    const output = proc.stdout.toString()
+    expect(output).toEqual('https_proxy')
+  })
 })

src/proxy.ts

@@ -200,7 +200,15 @@ export class ProxyBuilder {
       name: containerName,
       AttachStdout: true,
       AttachStderr: true,
-      Env: [`JOB_ID=${jobId}`],
+      Env: [
+        `http_proxy=${process.env.http_proxy}`,
+        `HTTP_PROXY=${process.env.HTTP_PROXY}`,
+        `https_proxy=${process.env.https_proxy}`,
+        `HTTPS_PROXY=${process.env.HTTPS_PROXY}`,
+        `no_proxy=${process.env.no_proxy}`,
+        `NO_PROXY=${process.env.NO_PROXY}`,
+        `JOB_ID=${jobId}`
+      ],
       Entrypoint: [
         'sh',
         '-c',