Add Dockerfile and entrypoint_config_from_env.sh

Update README as well

Author: giner

Dockerfile

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+FROM amazon/aws-for-fluent-bit:latest
+
+COPY entrypoint_config_from_env.sh /

README.md

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 # aws-for-fluent-bit-config-from-env
 AWS FireLens Fluent Bit image with an option to provide a custom config as an environment variable (`FLUENT_BIT_CONFIG_BASE64`) in base64 format
+
+This customization won't be needed if either https://github.com/aws/containers-roadmap/issues/56 or https://github.com/aws/aws-for-fluent-bit/issues/521 is resolved
+
+## Build and push
+
+```
+# Set vars
+aws_region=REGION
+ecr_domain="AWS_ACCOUNT_ID.dkr.ecr.$aws_region.amazonaws.com"
+docker_image="$ecr_domain/custom-fluent-bit:latest"
+
+# Build
+docker build -t "$docker_image" .
+
+# Push
+aws ecr get-login-password --region "$aws_region" | docker login --username AWS --password-stdin "$ecr_domain"
+docker push "$docker_image"
+```
+
+## Task definition example
+
+```
+{
+  "family": "firelens-example-cloudwatch_logs",
+  "taskRoleArn": "arn:aws:iam::AWS_ACCOUNT_ID:role/ecs_task_iam_role",
+  "executionRoleArn": "arn:aws:iam::AWS_ACCOUNT_ID:role/ecs_task_execution_role",
+  "networkMode": "awsvpc",
+  "cpu": "256",
+  "memory": "512",
+  "requiresCompatibilities": [
+    "FARGATE"
+  ],
+  "containerDefinitions": [
+    {
+      "essential": true,
+      "name": "app",
+      "image": "nginx",
+      "logConfiguration": {
+        "logDriver": "awsfirelens"
+      },
+      "memoryReservation": 100
+    },
+    {
+      "essential": true,
+      "name": "log_router",
+      "image": "AWS_ACCOUNT_ID.dkr.ecr.REGION.amazonaws.com/custom-fluent-bit:latest",
+      "command": [
+        "/entrypoint_config_from_env.sh"
+      ],
+      "firelensConfiguration": {
+        "type": "fluentbit",
+        "options": {
+          "config-file-type": "file",
+          "config-file-value": "/fluent-bit/configs/config-from-env.conf"
+        }
+      },
+      "logConfiguration": {
+        "logDriver": "awslogs",
+        "options": {
+          "awslogs-group": "firelens-container",
+          "awslogs-region": "REGION",
+          "awslogs-create-group": "true",
+          "awslogs-stream-prefix": "firelens"
+        }
+      },
+      "environment": [
+        {
+          "name": "FLUENT_BIT_CONFIG_BASE64",
+          "value": "W09VVFBVVF0KICAgIE5hbWUgY2xvdWR3YXRjaF9sb2dzCiAgICBNYXRjaCAqCiAgICByZWdpb24gJHtGTFVFTlRfQklUX0xPR19SRUdJT059CiAgICBsb2dfa2V5IGxvZwogICAgbG9nX2dyb3VwX25hbWUgJHtGTFVFTlRfQklUX0xPR19HUk9VUH0KICAgIGxvZ19zdHJlYW1fcHJlZml4ICR7RkxVRU5UX0JJVF9MT0dfU1RSRUFNX1BSRUZJWH0KICAgIGF1dG9fY3JlYXRlX2dyb3VwIE9uCg=="
+        },
+        {
+          "name": "FLUENT_BIT_LOG_REGION",
+          "value": "REGION"
+        },
+        {
+          "name": "FLUENT_BIT_LOG_GROUP",
+          "value": "fluent-bit-cloudwatch"
+        },
+        {
+          "name": "FLUENT_BIT_LOG_STREAM_PREFIX",
+          "value": "from-fluent-bit-"
+        }
+      ],
+      "memoryReservation": 50
+    }
+  ]
+}
+```

entrypoint_config_from_env.sh

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+#!/bin/bash
+
+set -euo pipefail
+
+base64 -d <<< "$FLUENT_BIT_CONFIG_BASE64" > /fluent-bit/configs/config-from-env.conf
+
+exec /entrypoint.sh "$@"